Having a real phone number that terminates on your PC is an extra feature for your GizmoProject software.
For adding a free incoming phone number to GizmoProject you may use one of the following methods.
Get a free number from IPKall
IPKall lets you receive free calls over the internet from any phone. The service provides a truly free phone number in Washington state area that can be forwarded to almost any SIP account. To make use of the IPKall phone number in GizmoProject you need to configure the service page with SIPphone SIP number and proxy information.
Area775 number from GizmoProject
The second option is to use Area775 service provided by GizmoProject itself. As name suggests the service gives you a free number in area 775 (as well as other area codes). The particularity of this service consists in the fact that incoming calls simultaneous ring computer and a regular phone. Answering a call on the PC is free but answering from the regular phone cost you $2 per call answered (if you have the free Area775 account).
Alternatively, people without a PC may reach you via an Access Number provided by a PSTN gateway such as these ones (dial first the gateway number, then the SIP number):