SJLabs has a free VOIP client for Windows mobile 2003 and 5.0 based Pocket PC devices. You can use it with any free or paid VOIP service provider like Vonage or Voipbuster.

Here are the settings for Voipbuster from their forum:

Right click on SJPhone. Select Option menu and then click in Profiles tab. Click on new button, type the name of your profile (e.g. voipbuster) and select Profile “Calls though SIP proxy) and click Ok button. A new window appears.

Finally, Right click on SJPhone and select Restart option. You user id and password will be asked.