FreePBX/Asterisk is a good alternative to this, if a pain to administer. Twilio seems great for a request/response Web app, but going much further than that seems like it'd be a challenge (by design; Twilio seems to emphasize that model and simplicity).<p>Rather than have Twilio hand off to your 'office number' for extensions and voicemail and such, you could just have both in the same place, which is the Asterisk approach.<p>I'd be interested to hear if anybody has pulled off extensions and voicemail using the Twilio system. I've never used it, but based upon a quick glance at the API, it seems like it'd be doable...also, thank you for the inspiration to do something with Twilio. Signing up tonight.