Currently Being ModeratedFeb 21, 2013 12:54 AM (in response to littlefire)
Try removing and reseating SIM and resetting your iPhone.
Reset: Press the Home and On/Off buttons at the same time and hold them until the Apple logo appears (about 10 seconds). Ignore the "Slide to power off"
Currently Being ModeratedFeb 27, 2013 1:15 AM (in response to littlefire)
I have this same problem since the 6.1.2 update. I get the 'No Sim' error at random intervals. The only way to restore service is to reboot the phone. Not good. I have reset the phone and the network settings but it is still happening. Not happy about this at all. I had no problems until the 6.1.2 update.