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.
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.