My phone (A13 I think.) keeps going to emergency call only mode, sometimes it also says no SIM inserted. The only way I've found to fix this is to turn the phone on and off again. I've ordered a new SIM from O2 and installed it but the issue persists. Is there anything I can do to fix this problem? Is it a hardware problem or a software problem or is it an issue with the SIM/network provider. 

Many thanks