Did you check whether Wi-Fi calling is on or off?
https://www.orange.be/fr/blog/5-choses-savoir-sur-les-appels-en-wi-fi
https://www.orange.be/nl/blog/bellen-wifi-5-dingen-die-je-moet-weten
What about the SMSC?
(They don’t have the exact steps for the Fairphone 3, or a generic guide it seems, but the SMSC number is the same, it can be set somewhere along these lines …)