I once called my mate and his wife answered as he was driving. I didn't know they were in Florida at the time so no, you don't need to do anything other than ring the normal number.
buildersmate - you could do that if you wanted but that wouldn't make any difference. That only applies if you are abroad and dialling a UK number. RJ You just dial the normal number.
Apart from the high cost of mobile telephones generally, I understand that not only is it exceptionally expensive to call a foreign number, but anyone receiving a call from outside the country they are in is charged at a premium. Thus the person you are calling in Spain will be charged a fee for receiving the call - you may be charged just the UK (i.e. domestic) rate but it would not surprise me if some companies will charge both parties a premium.