I live in New York, so I’m constantly losing my iPhone signal when I go onto the subway. Sometimes when I get off the train and I’m back in range of the phone still can’t find the network. Recently, my phone has randomly also been telling me “No Sim” even though it’s just been sitting on my coffee table or desk (and been in range).
To get my signal back I used to turn the phone completely off, and boot her back up. This solution was alright, but it took a lot of time and it occasionally took several attempts for my phone to actually find the network. So, I have new method. I simply:
Turning Airplane mode on disables the wireless features of the iPhone, and then turning it back makes the phone re-register on the network. I’ve had better success getting my signal back by toggling Airplane mode rather than rebooting the phone. Good luck!