If you are using OS2008, you already have rtcomm, just go to Settings/Control Panel/Accounts and add a new sip account.

Username: account_name @ voipdiscount.com
Password: ******
[Next]

That's all, you even don't need to go to advanced settings (but you may set the proxy to sip.voipdiscount.com if you want and the stun server, you can find the settings here). If you can't make any calls you will need to remove the iLBC codec, search the forums here for voipbuster or voipdiscount on how to do it... however it did work for me out of the box with voipcheap.com, which is basically the same company and sip server.