The British network "3" (that's their name!) actively encourage it, selling dedicated Skype phones and running a fairly heavy ad campaign promoting "free calls forever": http://www.three.co.uk/Company/3G_Network/Skype
Calls made via Skype don't eat up your data plan and aren't charged by the minute - they're actually free. I have one and it works surprisingly well, even in patchy coverage where voice calls break up. Whether it really will be forever, I don't know...
Yes, but that only works with their supplied phones. You can still use Skype on other phones, but you'll have to pay the bandwidth cost of £5 / month for 1GB.
Well, it's not that expensive. GB is much, isn't it? Here in Russia we pay around 1 ruble/MB which amounts to £20/GB, that's with recent price drops, which I consider decent.
There are no cellular providers like the "3", but on the other hand, phones are not locked to the operator as a rule.
As far as I know, that's because the Skype call isn't actually routed over the data connection, it transfers the voice via the existing voice network and the other end is hooked into a Skype gateway.
[Citation Needed] but I don't have time to dig one up at work
Calls made via Skype don't eat up your data plan and aren't charged by the minute - they're actually free. I have one and it works surprisingly well, even in patchy coverage where voice calls break up. Whether it really will be forever, I don't know...