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I am researching the voicmail message change procedure with the phone model you have named. It will take about 10 more minutes before I will post to this case again with results.
There is probably a phone system box that is in a back room, where your phone cable goes.
The voicemail procedure would depend upon what the maker and model # of that phone system box is.
The Panasonic phone model you named has no voicemail features in and of itself. It can access voicemail features, on some other device on your local phone system - most likely a voicemail card installed into a phone system box in a back room or equipment closet.
Is there a greeting pattern before the recorded message, when someone calls in and gets your voicemail? if so, what is the greeting pattern or phrase? we might be able to identify the voicemail system or card, from that greeting pattern.