Phone calls are going straight to voicemail

When somebody calls me on my Fairphone 2, they receive a free line signal, but are then re-directed to voicemail, while my Fairphone is not even ringing. I only get a text message afterwards, notifying me either of a voicemail message or a missed call.
I only recently got a new SIM card. I also tried my SIM card in the second SIM card slot, which didn’t solve the problem.
My OS is up to date: 21.12.0-rel.1
My call redirection is set only for missed calls and when my line is engaged.
I found older tickets regarding this issue - some of them closed, some of them still open. But none suggested what the solution to this problem might have been. Can anybody help?

Do you have another phone to try the SIM in? That could narrow down the problem to something either phone-related or more generally with the network/SIM.

Nope. Sorry, I don’t. :-/ And like I said: the SIM card is brand new. So I would I guess it’s very unlikely that the problem is caused by the SIM card.