New to the Windows Mobile 6, there is a SIP client built-in. It is tricky to set it up, but here goes the details:

1. You’ll need a copy of cabarc.exe, grab it at

2. Create your SIP config file, name it as _setup.xml

A sample config is below:

<characteristic type="VoIP">
<parm name="SIPSettings"
value="<provision key=’1032ab01′ name=’ProviderName’>
<provider name=’ProviderName’ />
<user account=’username’ password=’password’
<sipsrv addr=’your_sip_server:5060′ protocol=’UDP’ role=’proxy’>
<session party=’First’ type=’pc2pc’ />
<session party=’First’ type=’pc2ph’ />
<sipsrv addr=’your_sip_server:5060′ protocol=’UDP’ role=’registrar’/>
</provision>" />

3. From command line, go to folder where cabarc is located, copy the _setup.xml there, run

cabarc N _setup.xml

4. Copy the to your device and run it.

5. If you want to enable SIP client over WWAN (such as GPRS/EDGE/UMTS/HSDPA or 1XRTT/EVDO), simply go to your registry (you can use Resco Explorer or your favorite registry editor)

Go to the HKLM\Comm\RTC\AdapterTypes
Delete both the "1" and the "2" values that are here
Reset your device and Internet calling will connect over the data connection for your phone service.