I ordered one from my operator, Vodafone, for a good deal yesterday but I'm refusing the delivery today when it arrives, within the 14 day cooling off because of the disabled VOIP. I don't like features being disabled.
I know they might think it would affect revenue from standard calls but another operator, O2, don't disable VOIP according to these forums.
If you look at this is plain money terms the operator deal of 50pounds for the phone plus a certain amount of free calls is attractive compared to paying around 450-500 for a sim-free phone and stay with the same operator.
Overall in this case O2 seem the best because their network is better coverage than T-mobile or Three. O2's data rates aren't the best though there are bolt ons. O2 apparently don't disable the VOIP.
Just a shame they are not as good coverage as Vodafone and I would have the hassle of switching and the period where my number I'm moving is inactive for a few weeks.