I have the same issue as described in other thread (regarding a Fairphone 6 EOS) with my new Fairphone 6 with Google OS. For example today my husband tried to call me, and got “occupied", and I got an SMS from my voicemail telling me that I had two lost calls which I didn’t receive. Then, later, I tried to call him, and he answered after a few seconds just to have the conversation cut - while I had the calling tone all the time until it was interrupted because it had been calling for a minute or more, and when he tried to call me back, it took him minimum 20 sec to even connect to call. Later I found out that 4G calls had been enabled - BY ITSELF - even though I disabled it some weeks ago after not receiving calls correctly (it took eternity to connect to call, and calling numbers appeared “unknown” even though they existed among my contacts).
We don’t know if this is phone related, or related to some flaw in the provider, as this never was a problem with my much older Samsung Galaxy S9. It might be a combination maybe. Anyway it’s pretty sad to have this kind of problems with a phone I have in my hands less than a month…
Welcome to the forum and the community 
Settings like 4G Calling sometimes get changed during OS updates, so it’s always a good idea to check the most important ones after an update.
That means that the requisite information isn’t being sent to the phone along with the call, so something wrong that you can point out to the network operator.
I have recently noticed that (in general) 4G Calling is less of an option and more of a necessity with many operators. Fairphones (I think at least since the FP2) are all LTE / 4G capable. I would recommend that you request your operator to check that 4G Calling has been enabled on their network for your phone model and your line.
[Edit: grammar]
Thank you for your comment. I appreciate very much having this forum to go to, so that I can get a clue about it the problems I face are related to bugs and flaws in the Fairphone build or OS-version itself, or if it’s some problem with my phone only.
For example after a quick search it’s clear that the delay in showing content in certain apps when using WiFi is a common issue (that I hope can be fixed in an OS update decently soon), and this, that I’m not the only one having troubles with receiving calls as they should be received. Unfortunately this is not good advertising for more people to choose Fairphone, so I really do hope these issues are something which can be easily fixed in an update or two (or by the network provider), or else my next years being a Fairphone user will be a pain both for me and for the ones wanting to contact me…
Maybe your SIM is just faulty or too old.
I have en e-SIM. That’s new to me. I haven’t had it before. I’m open to that it might be related to that.
Could you tell which provider you are using? I understood from the other topic that you are not roaming, are you using one sim only? Could you check somebody else’s physical SIM in your phone? That could help narrow the issue to the phone being faulty or the settings.
I’m using Plussmobil (Norway). I’ll have to check with other SIM-card…
I think you ran into this problem, as I happened the same some years ago with FP3+:
https://support.fairphone.com/hc/en-us/articles/9779766652689-How-to-stay-connected-with-4G-Calling-2G-and-3G-shutdown
Phone operators are shutting down 3G systems as they are now focussed on 4G and 5G. If your phone is not able to use 5G or 4G it tries to fall back to 3G. As 3G is no longer there, it falls back to 2G and this could be the cause of your troubles: you’re simply using a very old mobile service.
I had the same problem, people called me, I received an SMS telling me I was called while I was not reachable but that was impossible as I was in an area with a good signal coverage. I also experienced very poor call quality, so poor that often it was impossible to understand what the other people was saying. Finally, while I was in a call the network connection was dropped, i.e. it was impossibile to use the network connection during a call.
If you go at the page I linked above you can check what operator in your country (if any) is supported by Fairphone 4G calling.
I think the only thing you can do is switching to a 4G supported operator or enable a 5G plan on your phone.
Ciao,
Max