My phone (GIGASET GS290, bought on your site in December 2021) is blocked.
The password to unlock the screen no longer works (the same password for several weeks). I restarted the phone and the PIN code no longer works. I tested on another phone, the PIN code works.
In order to be sure, remove your sim card and try to unlock your phone. If you manage to do it, then your sim is guilty. Otherwise, the phone may be responsible.