Statistically you probably know this person so we probably only need a first name.
Use the first 5 digits and put them into google - This allows network identification (eg 07973 is orange) and you could possibly 141 call him (get a friend or partner to do it - they probably know your voice) pretending to be from their network abuse department. Request he confirms he is the registered owner of the phone as documented on his system and get him to confirm his first name and say as confirmation you will reply with his surname (hangup or have fun here - your choice) . You should be able to work out the rest.
If he transfered his number it won't be his current network though, you'll just have to claim you're on behalf of you if it comes to it.