Press the mailbox (the right side of the left button). The list of missed calls that have not yet been acknowledged appears. Click completely through the list and thus take note of them all. Press the red handset to return to normal. Then the flashing should be gone and pressing the mailbox again shows the complete list of missed (but meanwhile acknowledged) calls.

ok, thanks!
I deleted the entire call list from the history, then it was gone.
The problem is that I have the answering machine on the Fritzbox and have to retrieve or listen to it there first.
This is feasible for the new incoming calls.