When you activate the warning tone, all other tones are deactivated. An incoming call is signalled by this brief tone. 

If the handset is in a call pickup group, a pickup call is also signalled by the warning tone. 

Activating the warning tone

Hold down the key for a few seconds. A confirmation tone is sounded. 

Press the Display key within three seconds. 

The warning tone is activated. 

Deactivating the warning tone

Hold down the key for a few seconds. A confirmation tone is sounded. 

Note:  The warning tone cannot be activated if the ringer was deactivated.