I had this problem too. And with a bit of trial and error found out that the reason for me was that the back cover sometimes pressed on the sim card.

You can also simulate this after removing the back cover by applying a tiny force on the top/top-left of the sim card slot while the device is running. This will cause the phone to lose the connection to the sim card. Maybe someone with a more knowledge about the sim card slot can tell us why this is happening?


My fix, or workaround, to this issue was to force a very tiny piece of paper under the sim card to the top left corner of the sim card slot. I used a tooth pick to force the paper into a correct position.