My phone 4 and after updating to ios 6 sometimes is unable reconnect to cellular signal and is stuck in searching -no service cycle. If I turn the phone off then on again it will reconnect and I will get 3 to 4 bars.
What I think is happening:
1- iPhone connects to cellular and uses 3G if the signal is good, however if the signal deteriorates it will fall back to edge.
2- sometimes this is what goes wrong and the phone is unable to connect to edge but instead of loosing my data connection, I lose all connection and get stuck in searching mode
3- a restart of the phone fixes this and an edge connection is made and maintained with at lest 3 bars as signal strength. If the signal gets better it switches back to 3G
What is frustrating is that you do not notice that you have lost you cellular connection until its too late and some important calls were lost.
I have restored my iPhone 4 to iOS 6 as a new phone and replaced my sim card with a new updated one from the carrier, yet the problem persists.