Incoming Landline Phone Calls That Just Show "Incoming Call" on Call Display and Aren't Missed Calls

[wireless]

I occasionally receive an incoming call on my Panasonic landline that shows nothing in call display but "Incoming Call". I don't ever answer and the call eventually disappears without ever being subsequently registered by my phone as a missed call. Just before the call drops the phone quickly flashes a "line in use" message. Normally the phone shows "Incoming Call" for a second and then having learned the number involved, flashes that number or say a province name.

Anybody have an idea what these peculiar calls are doing - beyond that they are spam?


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