I have found a really great forum for PDA's etc called XDA Developers - http://www.xda-developers.com/

Plenty of good advice and also software / firmware to make your supported PDA telephone SIP friendly....

Windows 6.1 Plus is the more friendly SIP OS and above.... it natively supports SIp within the standard dialler, no need for other applications... just dial the number ....

Regards

Kevin