Zoiper Free for all major platforms, Linux, OS X and windows
Source: snapvoip.blogspot.com
Attractel has released two versions of Zoiper IAX and SIP softphone: Zoiper 2.0 for Linux and Zoiper 2.0 for Mac Os X. (There is also Zoiper 2.0 for Windows.). These are all free versions. In addition to free versions, there is also a BIZ version available as well. The BIZ version has more options and you can compare those here.
Following is from release notes;
Linux, ver. 2.0:
• RTP media address is set automatically
• Implemented per account options (STUN for SIP accounts and codecs)
• Fixed problem with the audio
• Fixed bug: when dial is clicked and no account is selected, Zoiper crashes
• Fixed bug: when deleting last account, pressing OK crashes the phone
• Fixed creating of new accounts- empty name is not permitted anymore
