Panasonic KX-TGM420 Operating Instructions Manual page 88

Turning the first ring on and off
You can choose whether the first ring sounds when a
call is received.
"Yes" (default): The first ring for all calls will be heard,
including calls from blocked phone numbers.
"No": The first ring is muted for all calls. If this setting
is selected, the unit will never ring for calls from
blocked phone numbers.
1 [MENU] [#] [2] [1] [7]
2 [ ]: "One ring for blocked call"  [SELECT]
3 [ ]: Select the desired setting [SAVE] 
[OFF]
Viewing/editing/erasing call block numbers
1 [MENU] [#] [2] [1] [7]
2 [ ]: "Block a single number" or "Block range
of numbers"  [SELECT]
3 [ ]: Select the desired entry.
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