VoIP is becoming more common and practical. Essentially it relies upon transmitting audio data packets with low latency across broadband networks. Most of the VoIP services use the SIP protocol.
Kphone/Pi is a port of KPhone to the Zaurus kphone setup guide
TkcPhone is another Zaurus VoiP program tkcphone homepage
A third application that might be worth trying is Linphone, which also has a gui front end; this may only work for sharp-based (non-OZ) ROMS, however
[http://software.nautilus6.org/linphone/index.php Nautilus Linphone downloads