Strange problem with how my iPhone (4S) sees a friend who recently moved his number from his old network and device to a new network and iPhone 3GS

This is a weird one, so bear with me.
My friend recently got an iPhone 3GS with a new SIM in it (3 network in the UK). I helped him set up the iPhone and got iMessage working between my mobile and his temporary number (07429...)
In my Contacts I had two separate phone numbers for him: 07903... (his old number, on an old Sony Ericsson mobile with T-Mobile UK) and 07429... (the number that came with the 12-month contract I purchased for him; we called this his "temporary" number).
His old number (07903...) was transfered over to his iPhone with no problems. His iPhone has worked flawlessly.
But since that day I've had some really strange behaviour on my iPhone (a 4S, also on 3 UK): I removed his temporary number (07429...) from my contacts. But now when I try to text him at the old number (07903...) it bounces back at me as if the number doesn't exist. But if I try to text him at the temporary number (07429...) it works just fine and in my contacts and within Messages it appears as an iMessage-connect number.
So I tried deleting everything and starting from scratch. Still the same behaviour.
No one else is having this problem with him. I can call his old number (07903) just fine, I just can't text it.
I'm guessing that there's something with the way Contacts works with iMessage that's caused the problem. It's as if my contacts have updated properly under-the-hood (iMessage *does* work just not with the right number).
And yes, when I (or anyone) tries calling the temporary number (07429...) it's out of service. Which is correct. So it's not simply a case of *both* numbers being active.
So this is very peculiar.
In typing up this message it occured to me that I ought to try creating an all-new contact with some slight change in the name fields (e.g. include the middle initial) and see if I can get it to work properly. Then I ought to be able to change the name field (or just leave it alone, touch wood!).
Thanks in advance for reading this far. It's difficult to explain this problem succintly!


I've fixed my own problem.
In the end, the issue was with iMessage on the other phone, not mine.
While I had it connected to iTunes (the other phone, not mine), I could see that iTunes thought it was the temporary number (07429...). I turned iMessage off and back on again and the phone number in iTunes changed to the old (i.e correct) number (07903...).
Subsequently, texts from my phone to the other phone showed up as blue-bubbled iMessages.
Hope this helps someone!
Thanks for reading, if you did.