Troubleshooting - The handset does not ring. A speaker with a slash through it icon, a ZZZ icon or a star and moon icon are displayed.

When the handset does not ring and speaker with a slash through it icon is displayed the ringer volume is turned off.

Adjust the ringer volume.

While the telephone is ringing press the arrow up button ▲ repeatedly to select the desired volume.
or
To program the volume before a call, adjust the ringer volume on the handset;

Press [MENU # 1 6 0].
Press the arrow up ▲ or down ▼ button to raise, lower or turn off the ringer volume.
Press [SAVE].

When the handset does not ring and the silent mode icon or the Night mode icon is displayed.

Turn Silent or Night mode off.

Refer to answer How to use the night / silent mode.