The number indicates the active phone line. This indicator is shown only if the SIM card supports two subscriber numbers, that is, two phone lines.
Press and hold # to select Line 1 or Line 2 for making calls and sending text messages, you see a prompt Change the phone line if you have subscribed to this service. When the first line is selected, indicator 1 is shown in the standby mode.
When you remove the SIM card, the number indicator disappears from the display.