Calling specific contact makes phone turn off

Every time I call a specific contact the following happens: After several minutes suddenly the pone turns off so I restart it, enter my passcode and the SIM PIN. When I call the contact to continue our conversation the call ends. That happened at least 4 times now, so annoying.

The contact is not stored in the regular phone app but in the hidden storage of OpenContacts (opencontacts.open.com.opencontacts version 20.0). I start the call by clicking on the contact’s home screen widget.

Fairphone 3

/e/ version 0.11-p-2020082870302-dev-FP3

Status of security updates: 5 Aug 2020

Phone app version (com.android.dialer): 19.0