SIP Config – Your details will differ for other ISP’s!
Name: VOIP
Sip Server: sip1.exetel.com.au Port: 5060
User/Pass is your VOIP user/pass, check voip setup email from exetel.
Tools > Allow over 3G

Sip Dial Plan
Settings are pretty straight forward, country code 61, area code: 2.
Don’t check allow local calls, it didn’t work for me.

Windows Mobile
Goto Settings > Today and select Internet Calling.
On the Today screen it will show VOIP details, click it and set to use VOIP “Whenever”.

If not, then goto Settings > Personal > Phone and “Internet Tab”

Today screen should say VOIP “Selected” when everything is configured right. You can select it and it will altername between “Selected” & “Available”.
Then you can switch back to Touchflo 3D through Settings > Today.

Touch InCall Screen Tweak

I use this to stop the connection from dying during the call.
I checked the first 2 options whicher were:
Ignore light sensor
Ignore first screen off

You can then make calls using VOIP. Your phone will receive calls from both your VOIP number and your mobile number.

For a heap more information in regards to this, have a look at http://forum.xda-developers.com/showthread.php?t=299950