Er, I thought I'd answered that in my final paragraph! ;-)
If you receive any incoming call on a mobile phone in the UK (irrespective of whether it's from the UK or abroad or whether it's from a mobile or a landline) you will never be charged for the call as long as the phone receiving the call is registered to a UK account. (i.e. If Joe Bloggs from London visits the USA and phones his mate, Fred Arkwright, who's still in London, Fred won't be charged for receiving the call on his mobile. But if Hiram T Wartheimer, from New York, visits London he'll be charged for all incoming calls on his mobile phone, whether they're from his mother in New York or his friend in Brighton).
Chris