I find Twilio's calls to be very expensive. If you're serious about keeping costs down(and writing an app that is primarily voice), a better solution is Asteris

I find Twilio's calls to be very expensive. If you're serious about keeping costs down(and writing an app that is primarily voice), a better solution is Asterisk in the cloud with say, Adhearsion (https://github.com/adhearsion/adhearsion https://github.com/adhearsion/adhearsion). I don't think Twilio is a serious threat to the old telecom companies. But I am a huge fan of cutting out intermediaries.