TheKompany.com announced tkcPhone, a Voice over IP (VoIP) SIP-based application for the Sharp Zaurus PDA. You are able to make phone calls between computers without any fee's beyond the cost of the internet connection. You can make phone calls from your Zaurus to an actual telephone by making use of their arrangement with iConnect.
Here are some of the features:
-SIP protocol based phone
-Codecs: PCMU, GSM, G729
-Dynamic jitter buffer size
-STUN server support
-SOCKS support to allow calls from networks behind a NAT (we've tested as many configurations as possible, but you might run into a scenario that we are not familiar with, please let us know if that happens).
-SIP proxy support
-Digest/basic proxy authorization support
-Net2Phone compatible client
-Codec Negotiation - use of Offer/Answer model for SDP as described in RFC 3264
-Event Notification - use of SIP Events framework as described in RFC 3265 for presence, instant messaging, message waiting, configuration and other event packages
-Tested and works with the windows software phone www.estara.com