Microsoft CEO Satya Nadella has spoken out about Windows Mobile, saying killing it off was a mistake.

Microsoft had a tumultuous relationship with Windows Mobile for years, spending billions to deliver a product that would have widespread appeal and largely failing. The company even purchased Nokia’s phone business, only to sell it off a couple of years later.

In an interview with Business Insider, via The Verge, Nadella was asked what mistakes or decisions he regretted:

The decision I think a lot of people talk about – and one of the most difficult decisions I made when I became CEO — was our exit of what I’ll call the mobile phone as defined then. In retrospect, I think there could have been ways we could have made it work by perhaps reinventing the category of computing between PCs, tablets, and phones.

Microsoft has been working on Android foldable phones in recent years, although it has shown much of the same lackluster focus that contributed to the demise of Windows Mobile.

Only time will tell if Microsoft has any real chance of becoming a major player in the smartphone market, or if the company’s opportunity has passed for good.