Incoming calls from (Dutch) business numbers starting with 088 get diconnetected

Incoming Calls from 088- business numbers (Netherlands) are immediately diconnected by my phone. The caller hears one beep and then is disconnected. I get a notification that I have been called by this number, My phone doesn’t ring though.
I have no problem placing outgoing calls to these phone numbers.

The provider KPN coudn’t help me. I checked all possible settings on my phone. (blocked numbers, spam ID etc)

Well, maybe they didn’t want to be bothered, but I guess they are the best placed to help.

You should also inform official Fairphone Support. Even if there’s nothing they can do directly (and one never knows …) maybe they can lean on KPN.

Did you also check the MijnKPN settings?