OSS - Open Source SIP
Open Source SIP is a project created to foster the development of commercially viable SIP applications. Started in March of 2006, the Open Source SIP project is sponsored by Solegy, and draws on over six years of research and development.
http://opensourcesip.org/
Google PR:5

StanaPhone
StanaPhone is a free and simple internet telephone that uses a real US phone number and enables user to make and receive free calls to and from any phone anywhere in the US, Canada, and a selection of other countries. The software is free to download and free to use. It works with all PC headsets and USB internet phones providing the same or better sound quality than a regular household phone.
https://www.stanaphone.com/
Google PR:5