I used to work for Twilio. Their Twilio Client JavaScript SDK has supported WebRTC for two months now. You can use it to connect to the PSTN or SIP addresses. http://www.twilio.com/blog/2012/11/webrtc-for-twilio-client-...
As I said in another comment, I should know better than to expect simple fact-checking from a TC author. But somehow I'm still disappointed.