Why do I have no signal after 4s lcd repair?

This is bizarre I'm hoping that someone can help me here. I repaired a customers lcd/screen on an iphone 4s. The first the I nearly always check on iphones is if it still receives a signal. This did initially but after losing reception never regained a signal and still doesn't. Here in the North of England O2 were having a network problem for a few days at the time. So after taking responsibility for this fault and trying to determine what might be the problem I swapped the board with another 4s this particular one was an orange network blocked phone. I had no success so, I decided to tell my customer to return the 4s to O2 for a repair or replacement. The phone was backed up and restored prior to this. Here comes the weird bit O2 said they would not repair the phone because the imei number did not match the original phone and that this phone was blocked on the orange network. This could not be as I obviously made sure I returned the correct phone. For one the customers info was still on his phone. When the phone came back I checked the imei with the one on the sim tray and these matched. In settings/general/about/carrier it listed O2.

Any help regarding a repair or explanation would be greatly appreciated.