So I have been working on something pretty hard over the past week, just implemented for a business last night. They have been up and running for a couple hours today without a hitch.
In short, using Twilio API's, Bins, and a stateless service I setup on App Engine, I have turned Twilio into a full blown PBX.
You should know that Twilio offers SIP Registration and IVR, but no phone system features.
What my project does is add the following features (so far).
1.) Extensions and extension routing. The ability to transfer from a call back to an IVR/Menu
2.) Direct Parking Lots
3.) Busy Lamp Field Support
4.) Provisioning of phones against SIP registration or a small border element like Kamalio.
I am thinking of turning into a service that you can connect to your twilio account. Right now I am interested in "Can it do this or that" type questions because I can't think of anything that a FreePBX setup can do that I can't.