This article is a part of Poland Unpacked. Weekly intelligence for decision-makers
Can you build a technology company in Poland in just a decade—without raising millions from investors—that a global investment fund is willing to acquire at a valuation of around PLN 500 million (approximately EUR 117.5 million)? Mike Grabowski and Anna Lankauf have shown that the answer is yes.
Founded in 2016, Callstack grew its revenue from PLN 28 million (EUR 6.6 million) in 2021 to PLN 128 million (EUR 30.1 million) in 2024, while EBITDA increased from PLN 13 million (EUR 3.1 million) to PLN 47 million (EUR 11.1 million). In 2025, despite the change of ownership during the year, the company maintained double-digit growth, reporting revenue of PLN 151 million (EUR 35.5 million) and EBITDA of PLN 57 million (EUR 13.4 million). Yet Mike Grabowski, who chose to partner with Viking Global Investors, has set his sights much higher.
“If my ambition wasn't to build something truly significant, I wouldn't have stayed with Callstack. I want to turn it into a globally important technology company. And on a personal level, my dream has nothing to do with money—I want to see our logo on a Formula 1 car one day. Not as a sponsor, but as a technology provider,” says Mike Grabowski, CEO and co-founder of Callstack.
His target is to sustain annual growth of around 20% over the coming years. More important, however, is reshaping the company's offering around artificial intelligence. He believes that will position Callstack for an even faster phase of international expansion.
Interactive chart Global investor comes in, founder stays on
Viking Global Investors is a US-founded global asset manager overseeing more than USD 56 billion in assets—a scale matched by only a handful of European investment firms and far beyond the reach of most Polish financial investors.
Given its size and international reach, Viking typically focuses on the world's largest capital markets. Even so, it invested USD 50 million in Humaneva, part of Poland's Neuca Group, in 2024. The following year, it acquired a majority stake in Callstack in a deal valuing the company at around PLN 500 million (approximately EUR 117.5 million).
Among those who sold their shares were brothers Tomasz and Piotr Karwatka. They helped establish the company and invested several tens of thousands of zlotys in its early days, and remained active advisers throughout its growth. Their stake was worth more than PLN 100 million (around EUR 23.5 million). A stake of similar value belonged to Anna Lankauf, who also exited the business and has since focused, among other things, on investing.
Mike Grabowski, who owned a stake of several dozen percent, chose to stay. Having received dividends over several years, he had already achieved financial security and felt no pressure to sell his shares.
“Opportunities to partner with an investor like this are rare. Viking wasn't looking to buy the company simply to absorb it. They believed in our business model and our track record. They want to grow the company with me, they have ambitious goals and a bold investment approach. We clicked from the start, so I decided the opportunity was worth the risk. For me, it's a once-in-a-lifetime chance,” says Mike Grabowski.
Powerful fund will help—but won't run the business
Grabowski says private equity has acquired both a good and a bad reputation. So far, however, his experience has matched his expectations perfectly. On the other side of the table is a lean, agile team with considerable autonomy—very different from firms governed by layers of investment committees. That structure allows decisions to be made quickly, fitting well with the way Callstack operates.
“Viking supports us in making strategic decisions, is open to new initiatives, and gives us access to valuable relationships and growth opportunities. At the same time, you have to be prepared for greater financial discipline and more demanding reporting requirements. For us, though, it wasn't a major change. We were already operating with a supervisory board and a framework of monthly and quarterly reporting,” says Mike Grabowski.
Although Europe and Asia are gradually becoming more important to the business, the United States remains Callstack's core market, accounting for around 90% of revenue. Since the change in ownership, the company has also seen its profile rise among prospective employees, significantly expanding the pool of talent available to it.
“Let's be clear: no investor is going to build your US business for you. That said, a partner like Viking can make the journey much easier. Simply watching how they operate gives you a much better understanding of how business relationships are built in the United States. And while many prospective clients already know either Callstack or me personally, having an investor of Viking's stature behind us changes the dynamics from the moment a meeting begins. Viking opens a lot of doors—but it's still up to us to succeed once we're through them,” says Mike Grabowski.
Callstack bet on React Native
Callstack is a software engineering company specializing in mobile application development using Meta's open-source React Native framework. For years, it has been helping shape the global ecosystem built around the React library, which has grown over the past 15 years from a small community into an ecosystem of millions—largely without any dedicated marketing.
“React stands out because of its declarative approach. Instead of manually defining every step needed to update a user interface, developers describe what the interface should look like. That makes it much easier to reuse the same components and business logic across different platforms—whether web, Android, or iOS. As a result, developers can focus on the end product and business logic rather than the implementation details of each platform,” explains Mike Grabowski.
He acknowledges that, after so many years, React now faces credible alternatives. Its key strengths, however, remain its maturity, widespread adoption, and strong representation in the training data of large language models. That, in turn, reinforces its position as one of the technologies most effectively supported by AI-powered software development tools.
Comprehensive support for large enterprises
From the outset, Callstack has focused on helping large organizations navigate digital transformation. When a company wants to adopt a new technology but lacks the expertise, engineering talent, or implementation capabilities, Callstack provides both the technical solution and the specialist know-how needed to make it happen.
“Today, we primarily work with companies that have already chosen React and React Native. We help them migrate to the technology and continue developing their existing applications. Our offering is end-to-end—from strategic consulting and prototyping, through implementation, to training and long-term product development,” says Mike Grabowski.
Increasingly, the company also works with organizations for which React Native is not yet an obvious choice. It helps them assess whether the technology is the right fit and prepares them for a successful transition.
Mini interview
The clear source of Callstack's success
XYZ: Where did React's popularity come from?
Tomasz Muter, co-founder of Droids On Roids and founder of Heilo.io: React is one of the world's most widely used technologies for building web application user interfaces. Facebook created it because increasingly sophisticated web services—with constantly changing content such as news feeds and advertising—were becoming harder to develop and maintain using existing technologies.
React's success today is not simply the result of being the technically superior solution in every situation. Much of its strength comes from network effects. Companies choose React because it's easy to hire developers with React expertise, while developers learn React because there is strong demand for those skills. That, in turn, has created an ever-growing ecosystem of educational resources, tools, and ready-made solutions to common problems. The cycle has been reinforcing itself for more than a decade.
Its widespread adoption also makes it far more likely that the technology will continue to evolve for years to come. That matters to large enterprises. Even widely used technologies can disappear almost overnight when they depend on a small group of enthusiasts who eventually move on.
How does that compare with React Native's popularity?
In 2015, Facebook brought the React philosophy to mobile development by introducing React Native. It addressed one of the industry's biggest challenges at the time. Companies needed a website as well as separate iOS and Android applications, and each platform typically required its own development team, specialized expertise, and independent product roadmap.
Maintaining as many as three teams working on different versions of the same product was expensive, organizationally complex, and slowed innovation. That environment fueled the growth of companies specializing in native mobile development, including Droids On Roids.
React Native was designed to simplify this model by enabling teams to share skills, business logic, and portions of code while maintaining mobile applications within a more unified engineering organization. In its early years, React Native applications were easy to spot—they lagged behind native apps in several respects. Even so, many companies, particularly startups, were willing to accept those trade-offs in exchange for a faster and more cost-effective way to launch across all three platforms simultaneously.
What are its strengths and limitations today?
The new architecture has removed many of React Native's historical limitations. In addition, React is particularly well suited to AI models and autonomous coding agents because of the enormous amount of publicly available code, documentation, libraries, and documented solutions built around it.
That said, applications that depend heavily on device-specific capabilities—requiring extreme performance or exceptionally deep integration with iOS or Android—may still justify native development.
By contrast, our decision at Droids On Roids to build around Flutter was driven by the need for greater control over the visual layer and rendering across platforms, from mobile and desktop to the web and embedded systems. It also reduces dependence on the native interface components of individual operating systems.
Why was Poland's Callstack able to build a global business around React Native?
From the very beginning, the company committed almost entirely to what was then an emerging technology. But the real key to its success was how it built its market position around that decision.
Most software services firms begin by looking for customers that need applications, competing on price, marketing, delivery times, and project portfolios. Callstack went much deeper. It became actively involved in developing the React ecosystem itself, as well as the tools used by developers and software architects building large-scale digital products.
That is especially important for enterprise clients. Large organizations cannot choose technology partners solely because of an attractive presentation or a proposal that is 20% cheaper. Their priorities are security, stability, and reliability. An application failure can cost millions in revenue, damage a company's reputation for quarters, and drive customers away. In that context, reducing risk becomes the greatest value a technology partner can offer.
Callstack has positioned itself as the safest choice. One of its distinguishing features is the open-source software it contributes free of charge. What the company monetizes instead are its expertise, reputation, and trust—assets that cannot be bought through advertising or replicated simply by hiring more engineers.
It is also worth noting that two Polish companies—Callstack and Software Mansion—are among the eight founding members of the React Foundation. That is a powerful endorsement of their standing, expertise, and relationships within the ecosystem. The two companies joined global technology leaders including Amazon, Expo, Huawei, Meta, Microsoft, and Vercel.
Founder, Heilo.io
AI is driving changes in the offering and strategy
A new strategic direction recently adopted by the company is the intersection of artificial intelligence and React Native. It is developing several complementary products. Among other things, it is creating libraries that allow AI agents to build applications with increasing autonomy.
“Agent Device enables them to run and test applications on devices – both locally and in the cloud. At the same time, we are working on our own AI model specialised in React and React Native. In this domain, it is intended to be a cheaper and more efficient alternative to general-purpose models,” says Mike Grabowski.
After being acquired, Callstack itself now has a strong appetite for M&A (mergers and acquisitions). The company is looking for businesses with established positions in adjacent technology areas, strong R&D teams, or a leading role in the React community.
“We are particularly interested in the areas of AI and React Native, but value could also come from entering a new geographic market, having an effective sales operation in the US, or gaining access to another technology that fits our profile. We are continuously scanning the market,” says Callstack’s founder.
Founder returns to the role of CEO
The changes in the company’s offering are linked to Mike Grabowski’s return to the position of CEO in July. It was a return after a long break, as he had held the role only during the first three months of the company’s operations. He replaced Krzysztof Lis, who became an adviser to the management board. Grabowski acknowledges that his return was not driven by a single factor. Several developments happened to coincide.
“We started modifying our go-to-market strategy, the AI market is changing rapidly, and there are increasingly more decisions in our business that are strictly technology-related. I concluded that, as the founder who has been responsible for technology for years, I could bring the most value by leading the company as CEO. Staying close to the team, the technology and the market. The leadership team we have built gives me the confidence to step into this role,” says Mike Grabowski.
He explains that, in practice, it is not so much his role that has changed, but rather the function of the CEO at Callstack. He continues to set the strategy, oversee key projects, set the pace, inspire the team and drive the company forward.
“This is a move towards a more founder-led organisation. It means I am working more directly with the team. Fewer meetings focused on alignment, and more task-oriented work. Organisations of this kind are able to create and execute strategy faster. This is particularly important now, as the transformation driven by AI requires rapid response,” explains the entrepreneur.
Expert's perspective
A remarkable achievement by a Polish company
Callstack’s growth period largely coincided with the boom in IT services. At the same time, geographic distance became culturally and technologically less relevant. There was therefore no longer any obstacle preventing a Wrocław-based company from building a successful presence in the US.
The right timing was, however, primarily about the product. In my view, React Native is an embodiment of the need for standardisation and well-understood simplification – one technology for multiple platforms. This is particularly important in an era of extraordinary growth in mobile applications.
Callstack also perfectly identified the moment when mobile applications began giving way to the next technological wave, one that requires standardisation and easy scalability. That wave is agentic AI. The company’s pivot in this direction proved to be a bull’s-eye. Companies already working with Callstack now have a natural partner on their journey towards modern artificial intelligence-based solutions.
The scale of growth of technology companies in the US – incomparable with Europe – and the fact that React was created and gained particular popularity there support Callstack’s decision to pursue this direction. The company has achieved exceptional success in this area, which is not typical of Polish businesses. Other examples worth mentioning include Molecule.one.
Callstack is also benefiting from legal factors. These include React Native being based on one of the most open and transparent open-source licences: MIT. Moreover, compared with a Europe that is increasingly heavily regulated, Callstack operates in areas that are generally not subject to direct regulation. For example, the AI Act – broadly speaking – creates some challenges for organisations using AI agents. However, this applies only to the commercial use of products or infrastructure provided, for example, by Callstack, rather than to the way these products or infrastructure are delivered.
From Poland, it is possible to build a business in the US
Although Mike Grabowski considers the US market to be critical, he is moving from Wrocław to Warsaw rather than, for example, San Francisco. He is also recruiting primarily in Poland, although he is not limiting himself to the country. For a team that already numbers more than 200 people, he is looking for both IT specialists and managers.
“From the very beginning, we have been building a global business. Throughout our years of operations in the US, we have not felt that building it from Poland was a barrier. I also do not believe that a permanent move to the US is a prerequisite for further growth,” says Callstack’s co-founder.
Being present on site matters, which is why he travels to visit clients throughout the year. As a result, he does not work from one fixed location.
“In 2025, I travelled around 200,000 km and took 45 flights, most of them long-haul. Our clients are spread across the entire US, so more important than having a single address is being ready to be where we need to be, when we need to be there,” emphasises Mike Grabowski.
Key takeawakes
- Growth on a dream trajectory. Founded in 2016, Callstack increased its revenue between 2021 and 2024 from PLN 28m (EUR 6.5m) to PLN 128m (EUR 29.7m), while EBITDA profit rose from PLN 13m (EUR 3.0m) to PLN 47m (EUR 10.9m). In 2025, the technology company maintained double-digit growth, reaching PLN 151m (EUR 35.0m) in revenue and PLN 57m (EUR 13.2m) in EBITDA. The company aims to maintain growth of around 20% in the coming years. However, its greater priority is rebuilding its offering around artificial intelligence. In this way, it aims to prepare the business for even faster expansion in the future. Planned acquisitions are expected to support this accelerated growth.
- Conquering the US with a global investor. In 2025, a majority stake in Callstack was acquired by Viking Global Investors at a valuation of around PLN 500m (EUR 116m). It was the US-based asset manager’s second investment in Poland. The fund manages more than USD 56bn in assets. Mike Grabowski remained the company’s sole individual shareholder. “We had good chemistry, so I decided to take the risk. This is a unique opportunity for me,” explains the entrepreneur. He stresses that no investor can build a business in the US on a company’s behalf. However, an investor can provide significant support – and that is the case here.
- React Native technology specialist. Callstack is a software house specialising in building mobile applications using React Native, an open-source technology developed by Meta. For years, it has been helping shape the global ecosystem built around the React library. The technology makes it easier to develop applications for several different platforms simultaneously. Callstack supports large organisations that have already adopted React and React Native in migrating and expanding their use of these technologies. It provides comprehensive support, from consulting and prototyping, through implementation, to training and long-term product development. The company’s new growth engine is artificial intelligence – specifically, among other things, its own AI model specialised in React and React Native.