Why does my phone say "SIM card registration failed"?

The error message "SIM card registration failed" indicates that the phone is having trouble reading the electronic information encoded on the SIM card. This problem is often simple to solve.

A SIM card is a small integrated circuit embedded in a plastic card that identifies the individual subscriber attached to a particular mobile phone. Sometimes the SIM card or the tray in which it rests gets dirty, resulting in an error message. If this is the case, wiping down the SIM card with a clean dry cloth and blowing away any dust may be enough to correct the problem. If it does not, the next step is to insert the SIM card into a different phone and see if it works there.