Hi and welcome to the community and the forum.
I think you should notify both Fairphone support (see #contactsupport) and your network operator (“carrier”) about this, it’s highly likely that one or both of them needs to make a correction.
Somewhat similar case here, though in that case the problem was more specifically about 5G.
You might try setting your phone, temporarily, to prefer non-5G just to see if that helps:
Settings > Network and Internet > Mobile network > Preferred network type
set to “4G/3G/2G”
(These settings are principally about the mobile data connection, but do have a bearing on the voice calls too.)
It might be helpful if you could indicate the name and country of your carrier.