FlyNumber provides virtual numbers all over the world and can work with ANY VoIP provider.

If the Provider has a SIP uri ( that isn't firewalled or limited) we can send the calls there at no additional charge

You can easily set up our service with Callserve. They've provided us their SIP URI to make things easier for you. You enter the username with Callserve at the signup form and that's it. 

Callserve
Callserve
http://www.callserve.com/