At its height, Skype – a telecommunications app founded by Scandinavian entrepreneurs Niklas Zennström and Janus Friis, and Estonian engineers Ahti Heinla, Priit Kasesalu, Jaan Tallinn and Toivo Annus – had 560 million registered users. In 2005, just two years after its launch, the app was acquired by eBay. Later, a majority stake was sold to private equity firm Silver Lake. Microsoft then acquired Skype in 2011 for $8.5 billion. While it was revolutionary in its time, Skype's journey has been full of ups and downs. Now that Microsoft has rival Teams, the future of Skype is uncertain. CNBC explores the company's past, present and what's next for the company.