LoudHush is a softphone application. It functions as an IAX client for the Asterisk PBX. With LoudHush you can initiate and answer, hold and transfer voice calls using your computer.
LoudHush is written in Cocoa/Objective-C and integrates nicely within the OS X environment.
LoudHush is distributed as shareware software. If you run it without a license, it will quit five minutes after being launched.