I have read the previous comments on this and as I was in the process of purchasing a Jas Jar with the view to using VOIP I wanted to get to the bottom of it.
After a number of discussions with various Vodafone staff I can report the following.
They say that they simply dont support VOIP on their 3G data network.They are not blocking it...they say they simply dont support it because of "latency" problems with it and that when they roll out faster systems that this will hopefully disappear.
I suggest that their wording is rather sloppy and has giving room for many interpretations some of them quite wrong.
So there is no breach of contract situation or risk of unseen billing as some may have suggested.
I guess in the long run it pays to get it from the horses mouth.
Cheers