dam! seems the TnCs have changed since i got my contract.
heres what u need to know then:
If you choose a Pay Monthly plan which includes internet on your phone, you'll need internet coverage, check it at t-mobile.co.uk/streetcheck. Remember that you can only use internet on your phone in the UK and you can't use your phone as a modem or use internet on your phone for peer to peer file sharing, instant messaging or making internet phone calls. *Internet on your phone comes with a fair use policy of 1GB a month. We'll monitor how much you send and receive each calendar month so that we can protect our network for all our customers. If you use more than your fair use policy amount, we won't charge you any more, but we may restrict how you can use your booster, depending on how often you go over your amount and by how much. If you remove your booster and then use internet on your phone you'll automatically be charged our daily rate, see t-mobile.co.uk.
seems that vodafone allows voip:
15. If you are on an unrestricted data price plan or you have an unrestricted data bundle, you are permitted to use the mobile internet for:
a. calls over the internet (voice over IP); and
b. Peer to Peer Communication services.