my iphone 5 will not activate with my replacement sim (original damaged)

Hi All, Have a problem with my grandsons iphone 5 on tesconet. His original sim stopped working and I requested a replacement sim from Tesco, who sent it to me saying it was ready for use with the same number.When I put this into the phone it says it needs to activate and that the sim is not of the correct provider, even though the phone is apple unlocked. I have tried a couple of other tmobile sims in the phone and they work ok, and the original tesco one worked ok for 3 months. I have tried reloading the software and just about everything else on line, also I tried the new sim in another mobile and that works ok, it doesn't even ask to activate, just works straight away, any ideas?

Thanks

Alan