this video compares an orange (+vodafone) locked phone vs a contract-free outright phone. in the orange nokia n95, they've disabled the internet telephony features in the UI so that you cannot make use of IP telephony applications that integrate with series 60 phones.
i don't know whether that also applies to nokia n95's purchased with O2, T-Mobile or 3.
[UPDATE: vodafone officially comes out to say that it is for our own good that N95 does not have VoIP enabled. because otherwise we'd be ripped off and incur more expensive phone charges! thank god for vodafone's babysitting otherwise we'll be paying those ridiculous mobile bills. oh hang on ...]