There are too many factors stacked up against VOIP over cellular data. Plus the latency is too high. Have you ever tried making a VOIP call over Edge, 3G or HSDPA? The latency is terrrible!
I use Fring every day or two to chat to my brother in London (just takes an sms to tell him to log in), and with 3G its perfectly clear, GPRS varies from kinda usable to terrible and of course Wi-Fi over ADSL is crystal clear.