Microsoft purchased tech and people from AT&T to help boost Azure's telco capabilities as a cloud solution[0].

I haven't followed what's happened since it was spun out, but I know that Network Cloud was heavily OpenStack based so I'd would be interesting to see how that may have changed but I no longer work in the Telco space either.

[0]: https://azure.microsoft.com/en-us/blog/improving-the-cloud-f...