We evaluated Twilio, but decided against it due to lower call quality. Their gateways are virtualized in the cloud and this can lead to timing issues or hollow/tinny sounding audio. Twilio also lacked their SIP endpoint (not sure if this is still in beta) at the time we started building Close.io, which would have allowed us to do native calling. We ended up working with Plivo (another YC company) and have been happy with our choice.