I think this is an appropriate time for me to mention the service calltrackingmetrics.com. We offer a service, that allows you to manage the call flow as well as track many other features of a call - but in this case I believe the major selling point is we have people who answer the phone and work really hard to make sure your phone numbers port over to our system without issue. The Twilio team also helps make sure this process is exceptional.

The target audiences are a little different, but you can replicate the functionality of a Google Voice number pretty easily. You can forward voice calls with a little bit of XML - https://www.twilio.com/labs/twimlets/forward for example - or use something like SendHub to view/reply to incoming SMS messages.

You're right - they're essentially a step removed, although I'm working on webvbx.com which would be an easier on-ramp to using twilio as a hosted service.