SIM908C

voltage is less. Check module datasheet for voltage and current requirements. I think you need around 4.2v for module to keep up. and Sim908C need a good battery. what is the mAh of your battery?

Thanks for replying.
Though the battery is rated to 3.7v, i measured the terminal voltage as 4.1v. during pulling down of pin 13, the voltage come down to 3.9v. datasheet says the required range is 3.2 ~ 4.8v. my battery is of 1200 mAh.

Module takes maximum power when its searching for network or communicating over network. So you need monitor voltage (preferably on Scope) when module is turned on. Check if there is any voltage drop.