Yes, it is.
If you can read Portuguese check my Orkut community "Voip no Celular Smartphone PDA":
orkut - Login
In general you can do mobile voip in 3 ways:
- callback (triggered by SMS, WAP, ANI or provider application)
- calling a gateway (a regular phone number that will work as a bridge)
- your mobile working as a voip device (itself or a softphone running on it and data sent via WiFi, GPRS, EDGE, etc.)
Some nice links for mobile voip:
fring - Mobile VoIP application for cellular handsets
Free softphones for Windows, MAC, CE, LINUX
www.vyke.com
MINO: Mobile International Network Operator
Telme - Welcome to Digital Network Marketing
Tokiva | Go ahead, just call
Webacall.com - pc to phone call, internet phone call, cheap phone calls, pc to phone
Hampton Software Limited - VoIP & PalmOS Specialists