Re: Forward thinking?
Vodafone intercept and actively prohibit SIP VoIP over their data network, whether under their own brand, or under the brand of a MVNO such as Talkmobile.
We had that for a long time. SIP worked just fine on O2 but not on Vodafone. Had a wee word with our phone reseller (small company) and they fixed it right up. So it can be done. I doubt you'll get anywhere speaking to Vodafone directly, though. I know I didn't..
Anyway, our own SIP PBX is now usable on our Vodafone SIMs.