The only solution that works for me is to bounce the phone or power cycle it. This is the only solution that works but is very painful especially when you need to use the phone right away.
A. Chances are your phone has this display on screen (see below).
I have had my share of quarrels with the service provider and have been issued a different SIM card but that has not resolved anything. Not a single week passes that I don't get this "No Service" plague. As I'm running out of suspects, I couldn't help but point to the phone as the likely cause.
This problem can be attributed to the switching between 2G and 3G modes which the iPhone has difficulty handling. Could also be a bug in the iOS software. Like most of you I'm using an iPhone 4S with the not so latest iOS 5.1.1. Upgrading to iOS6 would be disastrous but that is an entirely different issue altogether.
To resolve this "No Service" issue, here's the workaround that I've done: turn off the SIM PIN. To do this, you need to go to Settings » Phone » SIM PIN » (tap) SIM PIN OFF.
After putting this setting, the "No Service" displayed on the phone recovered to regain the carrier signal and the phone could be used for its primary purpose of calling and sending SMS.
I have had this setting for over a couple of weeks now and the "No Service" phenomenon has not returned (and hopefully will not).
Turning off the SIM PIN is not very secure but that is what makes the phone "usable". I have been on the hunt for a working solution to this problem and found that many share the same problem or something similar. While this may not solve everyone's issues, I'm hoping that it will work for many.
