OK, so I have been living with VOIP over 3G for a little while now. It is working fine providing you have a good 3G connection. How you know you have a good 3G connection is a mystery to me but I guess the signal strength is a factor. The service provided by Exetel is very good and the price – well it almost unbeatable.
The calls I have made have almsot all been very clear with just a little bit of garble now and then. Last week I had an 8 min phone conversation and the call was very acceptable, especially given the whole thing cost me on my calulation about 50c (data of 2MB per min and VSP cost of 10c).
I think the major issue still is the fact that SIPdroid only supports the G711 codec. With luck the recent bounty proposed on SIPdroids site for the development of low bandwidth codecs (lets hope G729) will help make this an even better experience (an even cheaper).