voip.ms provides a hosted PBX solution, basically (don't need a DID, since accounts are free to register, but without a DID or SIP URI, you'd have no way to call into the system). You can set up all the routing stuff yourself through their web interface. For example, for our convention we set up an automated menu that can take voicemail and forward it via e-mail.
You can create an account for free and poke around in the web interface for configuring all this.--
Developer: Tomato/MLPPP, Linux/MLPPP, etc »fixppp.org