Here is a list of some of the products through which you could use the mobile phone to make calls over the IP network (GPRS or WiFi) instead of GSM, in other words VoIP over mobile (though cheaper- not all are 'free'):
1. Mundu Mobile, by Geodesic.
2. Skype on Mobile, by iSkoot.
5. MobileTalk, by Packet8.
Unless its over a WiFi network or a 3G/4G GSM network, most of these solutions would suffer from slight delays due to the nature of mobile data networks today (network latency.
Though not exactly pure mobile VoIP, JAJAH is an innovative solution that would probably have better results than the ones mentioned above simply because, though you can make free calls even over mobiles, at least the last loop on your end of the call would be over the GSM network (though its a VoIP at heart).
So wish you all happy cheaper calls!