iPhone repeatadly calls number while locked

I have a weird problem with my iPhone X: from time to time it keeps calling the same number (an old one not in my addressbook anymore) over and over for about 5 min. Last time I had 13 calls to this number and the Recents list say "cancelled call” (about 2 every minute).

This has happened 3 times now, sometimes when the phone is in my pocket, sometimes when it's on a desk. Siri is turned off and face id is active so I can’t understand how its possible even...