SIP Application for MAC OS X

It's been very tiring to keep your MAC working on some softphones. You need to make sure that you have the most compatible softphone for your Mac. Nomado has released MacPhone a new SIP application for MAC to ease the problem of all Mac SIP users. No need to configure. Just request the application from Nomado Team, then download, you can start calling right away.

This has a reasonable write up Download iSoftPhone for Mac - VoIP virtual internet phone. MacUpdate Mac Software Downloads
or you could buy an adaptor and use a standard phone and one of the free SIP sites.