http://www.xten.com/index.php?menu=products&smenu=xproppc This is the pocket pc version of the xten client by vonage. All you need to do is transfer the PC softphone setting to the Pocket PC softphone.
If you want to try a free VOip softphone use Skype. I can test it with you since I have an account which I share by putting it on my Dell Axim or my Laptop. I use this mostly when travelling since it one of the only ones that works even through firewalls. I jump into networks on the street and make my phone calls on my Dell Axim via wireless. A lot of open networks can be jumped into specially around residential condos in any downtown city. This way I don't pay for long distance cell bills when I travel.
http://skype.com
This client is also so cool that you can purchase timeblocks for a cheap 20 Euros and can then call real cell or land lines for mere cents a minute. Also even if you don't have a PDA or laptop you can stroll into any internet cafe anywhere in the world. Download the client and install it in the machine you are using and make phone calls to anywhere if you have bought one of those blocks of time. I believe Skype is not that big in North America but it is very big in Europe. All my Euro friends contact their families this way.
