Years ago I setup my account with, what at the time, was called SipPhone. They assigned a number that you entered into the hardware device, the number represented your unique account. The number they assigned my account was 1-747-xxx-xxxx. Any other users of the SipPhone network could call me using that unique number, but you could not use that number on a regular PSTN line.
Later, I was able to purchase a "call-in" number that was a standard PSTN number that anyone could call and it was routed by SipPhone to my hardware device.
SipPhone became Gizmo5 and now they focus on web based interfaces using user names but if you look carefully in the web interface you can locate the number that is assigned. Gizmo5 was just purchased by Google.
I hope that answers your question. If not let me know and I will be glad to help in any way.
Maybe this link to the Gizmo5 knowledgebase will help.
http://support.gizmo5.com/index.php?_m= ... id=0&nav=0