The firmware build on it was ending with .55 . I flashed a .69 Taiwan version on it and had the same problem.
I returned the phone and bought a similar z2 from Orange Romania . The the firmware build on this one is .69 personalized for Orange Romania and I still have the same problem , the phone reboots during some calls.
I couldn't find other people with the same issue and that seems strange since the 2 phones are totally different batch.
Can anyone give some clues on this isuee?