An XDA Developer forum member has posted details on how to configure and provision the built in VoIP SIP client embedded in the ROM of Windows Mobile 6.0 based devices.

First of all, you need to verify if VoIP support exists on your mobile device. To do this, you can either look for Internet Calling plugin on your today screen or open File Explorer and browse to the \Windows directory. Make sure you select to display all hidden files. Check if the following files exist in that folder:

ipdialplan.xml
dnsapi.dll
voipphonecanvas.dll
rtcdll.dll

If not, you can download a Cab file to install the VoIP SIP client on your Windows Mobile 6.0 device from here

After that is done, download cabarc.exe from here or here

Then download the _setup.xml file from here

Make sure to save and rename the file back to .XML extension

Replace the following details in the _setup.xml that you downloaded earlier with your SIP VoIP service provider settings:

_sip_provider_name_ with your provider name
_account_ and _password_ with your user name and password,
sip:_user@sip_host.com_ with your SIP address
_sip_proxy.com_ and _sip_registrar.com_ with your SIP proxy and registrar

Note: Add the port number (usually :5060_ against the proxy and registrar server name. For example, proxy01.sipphone.com:5060

Make sure the file is saved as _setup.xml

Then copy canarc.exe and _setup.xml to a folder lets say c:\voip

Open up the command prompt and navigate to c:\voip

Run cabarc N voiptobeinstalled.cab _setup.xml

Once the voiptobeinstalled.cab file is created in the c:\voip folder, transfer it to you mobile device and install it

If the SIP provisioning worked you should find a Prov0SIPSettings in the \Windows\System\VoIP folder on your mobile device.

If not, there might be a syntax error in the _setup.xml file.

Thanks to eluth from the XDA forum!

Update: SIP Config Loader- Free utility to automatically configure & load _setup.xml file for SIP VoIP on Windows Mobile 6.0 device