An open source SIP softphone SIP Communicator is available to download here:

http://www.sip-communicator.org/index.php/Main/Download

Details on how to configure a VoIP Advantage SIP account in SIP Communicator, click on the images to get the full size (and therefore readable) screen shot:

The key information is:
SIP id: your 7 digit extension number
Password: your VoIP Advantage password
Registrar: sip.voipadvantage.co.uk
Display name: Whatever name you want displayed
Authorization name: your 7 digit extension number
Server Port: 5060
Proxy: nat.voipadvantage.co.uk
Proxy Port: 5082
Preferred Transport: UDP
The rest of the settings the default selection will suffice.