Recently, when I make/receive call through iphone 4s, the signal is intermittent/move to "searching" mode. I move my sim card to another phone, no problem to make/receive the call. I reset the network setting in iphone, it doesn't work. I approach a repair centre; they say cannot be repaired. What can I do?

I dropped it. I have tried full restore; doesn't help. They don't tell me what is the problem.
I stay in Penang. Where can I go to repair?