Panasonic KX-TGM420 Operating Instructions Manual page 145

– turn on the new message alert setting.
After you answer the new message alert call, you can
listen to messages from that call (page 149).
Important:
• A new message alert is stopped 1 minute after the
unit starts to call. The unit will not retry the call
even if the call is not answered.
Storing a phone number to which the unit makes
an alert call

From the phonebook:
1 [MENU] [#] [3] [3] [8]
2 [ ]: "Notification to"  [SELECT]  [ADD]
3 [ ]: "Phonebook"  [SELECT]
4 [ ]: Select the desired phonebook entry. 
[SAVE]  [OFF]
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