Q: Always lost signal while calling having network problem

I have iphone 3gs 16GB factory unlocked Model no: MB715LL/A Serial Number: 80017BJW3NP IOS 6.1.3 firmware: 05.16.08 facing lots of signal problem always in searching network while calling please suggest what shall I do to increase signal of the iphone 3gs.

IMEI: 01 215600 729965 0

Earlier my phone was working perfectly fine since I have unlocked it and updated to IOS 6.1.3 with firm ware: 05.16.08 I am facing network problem if I put these phone on desk or in my pocket signal lost or showing searching I am unable to make any calls or calls drop suddenly in between because of network lost.

I have also check same Sim in another phone its working perfectly fine so this means there is no such Carier issues problem is with Iphone model.

Please suggest what step should I take for my iphone to start working smoothly.

Thanks in advance for your support.

Amit Khanna