Yes, to date Twilio has used Asterisk along with OpenSER. Asterisk has its place, as I was personally involved in Adhearsion (http://adhearsion.com) and now Voxeo Labs sponsors the project. The place for Asterisk today is not for providing large scale distributed telephony services. Simply the wrong tool for that, which Twilio has learned the hard way.

This is why Digium has created a new project: Asterisk Scalable Communications Framework (SCF - http://www.asterisk.org/asterisk/scf). Although Asterisk SCF is still in prototype stage and 12-18 months from a fully baked alpha. An interesting approach, but not here yet.

Folks from Twilio have confirmed they are trying to move away from Asterisk, I suspect to Freeswitch. While Freeswitch has better scale, it suffers from some of the same fundamental architectural issues Asterisk has for large distributed telephony applications.

Full disclosure, I am the VP of Innovation at Voxeo Labs, the group responsible for Tropo. Further, I was invited by Digium and attended their closed discussion and launch of the Asterisk SCF platform in Huntsville last spring.