I have the same annoying problem and I know another Fairphone user who has the same problem, too.
- calling others: no problems
- email/whatsup/appsstore,…: no problems
- using Firefox: no problems
- provider: enough money, etc.: no problems
-Calling me: impossible
I changed my SIM card to another mobile phone - no problems with receiving calls. Thus: it’s a Fairphone problem!
BTW: when I put my SIM card in slot 2 of my Fairphone, the card is not even activated.
How did you solve it?