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Did it ring formerly?
Does it still flash nowadays, instead of ringing?
What makes you think it is a programming error? if you have any information I'd like to know it, even if in brief, please.
That line may be in 'silent mode' or do-not-disturb mode. To disable do-not-disturb mode on your Panasonic phone station,
The abbreviation for do-not-disturb mode is 'DND' If
shows anywhere on the display (when you have line 1 selected) then it is a certainty that DND mode is enabled.
Select the line (Line 1)
then press the DND button on your phone station. That should turn Off the DND indication on the screen, as well as (perhaps) the light idication on or under the DND button.
(The DND button is located just to the right of the 9 button.)