While I'm a huge fan of Twilio as a service there are certain use-cases that they do not fit. It would be ill-advised to build a call-center operation based on Twilio, but it would be less so to use AT&T for the same. Twilio are a reseller based on networks larger than themselves, one of which I would assume is AT&T. I don't see them killing AT&T off in the near future.