If you are looking for a freeware SIP client that can run on your Pocket PC, SJPhone is recommended. It is a cross-platform program that can run on Linux, MAC, Windows CE and Windows XP. You can have configurations for different SIP providers and easily switch between these providers.
Upon installation, select Menu->Options->Profile tab. Choose to add a new ‘call via SIP Proxy’ profile. Most likely you’ll only need to change the SIP Proxy and the STUN tab according to your service provider settings. You will be prompted for username, password and caller-ID the first time your connect to this provider using SJPhone.
SJPhone won’t automatically establish a connection. You must manually make a connection (either Wifi or GPRS/3G) first before starting SJPhone. Audio will also be played to loudspeaker/headset. However, if your device supports the necessary APIs, there are tools to route the output audio to the earpiece, see this for more information.
Notice that if you attempt to install the WM2003 versions on WM5 or later, everything will appear fine until you receive an incoming call. The incoming call notification will still appear but there will be no options to Answer/Reject the call – just a ‘Hide’ button. This is probably due to the differences between the SHNotificationAdd API in WM2003 and WM5.
No version is available for Smartphone yet.
http://www.sjphone.org/preview/ (all versions)
http://www.sjphone.org/preview/SJphone-1.60-WindowsCE(preview)/ (Windows CE)