In my case, it was not the sim or antenna for sure. The tech guy has changed the antenna with a new one in front of me but it did not help. I also replaced my sim card, even tried 128 k sim card it did not help either.
Just for the reminder, my problem was; The phone was losing signal and was switching to no service when 3G is off. When 3G is on I was getting signal only if there is a strong signal (maybe a basestation nearby) and if I dont move. But when I was mobile (on my car) , I was facing the searching, no service, signal, searching again, no service again and then the signal.. Rebooting, restoring, updating.. 2 months 😟 Nightmare..
The tech guy told me that it was either antenna or main board problem. I wish it was the antenna, because it would cost much less but unfortunately it was the main board. He repaired the main board and it took him 5 days to give the phone back to me. Usually they do the small repairs such as antenna change in one day here. Whatever he has done, it worked, now its working perfectly fine. It has been two weeks so far and no problem at all. Not even one drop.
I hope it helps..