If you are using the GSM system with a SIM card, the provider of the SIM card may require you to activate it before it can be used. Typically they will ask for an IMEI number of the device that the SIM will be used in. Each panel's IMEI number is unique.
To find the IMEI number on the GSM panel, press 0, 1, then Enter. The IMEI number for the panel will then display on the screen, as seen below.