[Q] Xperia Z won't answer calls topic

Hi! I'm new to this forum.
I'm working as an IT manager, and have a user who has a Xperia Z. He have had problems with it since day one. The phone often does not respond when there is an incoming call, even when he uses BT handsfree and Sony SmartWatch 2.

The phone have been to service several times, replacing the screen one time, and charger port another time (the service company claimed it was broken), but I'm unsure if that means replacing the motherboard as well. We have even recplaced the sim card and tried with a twin sim card and the main card in another Z. We also tried backing up the phone and restored the settings onto another Xperia Z, and that one had no problems what so ever. We also tried backing up the other phone and restoring onto the phone with the problem, and the problem was still there.

Model number C6603
Android Version: 4.3
Baseband version: 9x15A-ACEFWMAZQ-30110082-51
Kernel version 3.4.0-g1b44cd5
Build number
10.4.B.0.569

Can anyone help? :)

Thanks in advance!