Not one hundred per cent sure about this but I'm pretty sure you just dial the number of the mobile. I received calls on my mobile in Menorca a few years back, and I dn't think the person calling me dialled anything other than my usual number. You will need a code number if you are dialling a land line. From Spain the the UK you drop the 0 from the number and put in +44 then the rest of the land line you are calling. You should be able to check the numbers you dial for Spain in the phone book.
If it's an English mobile, you just dial the mobile number. If the owner has enabled international roaming, it will ring in Spain. You'll pay for the UK part of the call and the owner will pay for the international part.
If it's a Spanish mobile, dial 0034 then the mobile number. You'll pay for the whole call ...