A company called mobivoip.net is providing a service that allows free VoIP between its users, from a Palm OS handheld to another. Some details are available on tamspalm.tamoggemon.com, including more specifics about the service.
The company also provides calls outside the net - to landline or cellular based telephones, but these are subject to charges.
This is an interesting software, since we don't usually hear much about VoIP on Palm devices. Most of the times VoIP is associated with Windows Mobile Pocket PC.
According to the developers, who posted a series of replies on the original blog post, the software requires 60kbps (30kbps up + 30kbps down) and a little less when noise suppression is used. Using with EDGE will not give good results as it has an ok download speed but not the required upload speed and will not quarantee the delivery of the packets at the required time.
Interesting to note that this is a good solution for markets where mobile operators offer a flat fee data plan. In other markets (such as most countries in Europe, and Australia/New Zealand) where data plans are charged a per megabyte fee, this may not be such an interesting program - unless of course you are using Wi-Fi (but again, in some places Wi-Fi use on hotspots can be as expensive).