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?
I think it is a quirk of Panasonic cordless phones. I have seen that happen over the years and there is some Caller ID information on my corded Nortel Vista phone but not on the panasonic. I can't be certain but that is the best explanation I have.
By "free fax number" do you mean you have a second, smart ring, number associated with your landline?
Do you have a fax machine sharing the landline?
If you call the fax number, does the Panasonic phone behave as you describe?
Can you try the star code to find out the last number that called you and see what it says?
Make sure not to use the call trace one unless you want it reported to the police.