...on the road. Aside from Google Voice, the same service also works with Google Talk, Google Apps, Skype, Salesforce, SIP, PBXs, and PSTN numbers. So calls could be diverted to a Skype ID as easily as to a Google Voice number, for example...
http://blogs.zdnet.com/Greenfield/?p=403
... Existing PBX Ordering and Contact Information SmartSIP server licenses allow connectivity to unlimited ITSPs, SIPPBXs, IP gateways or similar devices. SmartSIP client licenses are priced per endpoint and are quoted based on the total number...
http://www.evangelyze.net/brochures/SmartSIP%20Brochure.pdf