sim 800c doesn't get registered in network
It only tries to register when it has a SIM. 

To register, it must transmit.  

Either the supply voltage drops when it transmits and causes the processor to hang, or the RF is getting somewhere and causing problems.

I don't see a bypass capacitor on your power supply in, though there may be one on the module.

You may also need a large capacitor on Vbat to help with the current peaks when the module transmits.