SIM card rejected? Phone doesn't ring?

I have a very basic mobile (cell) phone, bought simply to make and receive phone calls. After a year, I started getting the message "SIM card rejected" with increasing frequency, requiring me to restart the phone. Of most concern to me was that I might be missing calls during the periods of rejection. I took the phone (a Nokia 3120) to my local Orange shop where I was told that the phone might be dying on me and that I would have to get a new one. I was not pleased!

A few more months elapsed and I found that the phone had stopped ringing, though the vibrating alert continued to work. I checked my phone settings to make sure that it was set to ring. Everything seemed to be set up OK, and the phone was still producing bleeps, suggesting that the hardware was working properly. Again, I took the phone to the Orange shop and was told I'd have to buy a new phone. Given my belief that the hardware was operational, I decided not to give up on the phone. I turned to the user manual and found how to restore the factory settings. Having done this, my problems disappeared. Note that the contacts list and text messages stay intact after the restoration. If you have a similar phone, I've copied the instructions below:

Restore factory settings
To reset some of the menu settings to their original values, press Menu, and select Settings and Restore factory settings. Key in the security code and press OK. Note that the data you have keyed in or downloaded, for example, the names and phone numbers saved in the contacts list are not deleted.

More troubleshooting information is available from Nokia.