X-lite is a softphone that is freely available from CounterPath.
You can download a PC or MAC version from the following links
http://www.counterpath.com/x-lite-for-windows-download.html or http://www.counterpath.com/x-lite-for-mac-download.html
After running through the installation and starting x-lite for the first time you will be prompted to enter you account information.
The screen should look similar to this:
Your account settings are:
UserID:
Domain:
Password:
Display Name:
Authorization Name:
On the voicemail tab put ### in as the 'Number to dial for checking voicemail'.
Then click ok and the system should register and be ready to use.