This is exactly what it looks like in my case as well and I have received an answer from my provider that my contract includes WiFi calling, there should be no limitations from the SIM card and there’s nothing to be enabled manually on their side.
In summary: It’s a problem on the phone.
I’ve tried WiFi calling using FPOOS as well as LOS, but it did not work.
Are there any technical (hardware) limitations that should prevent WiFi calling on FP2?
Or is it purely depending on the OS, so in theory it could be patched/fixed?
Well, as far as I have understood my provider, there are provider-specific settings required on the handset. This is why my provider supports only a few handsets with most of them coming with a provider-specific OS-implementation.
In theory, Fairphone could now implement these settings (in case the provider provides this information), which means a lot of administration work and warranty overhead in case the provider changes its setting. I believe this is the reason, why Fairphone has rejected my corresponding bugtracker report some time ago.
So for LineageOS it may be similar. Of course, in theory, people could provide code and settings for their provider to make this working out (for just their provider) but I think effort and outcome might not be as expected initially.
At least nobody claimed success in this forum yet.
The only thing coming somewhat close was installing Fritz!App Fon which could turn the phone into a wireless landline phone connected to the (at least in Germany) well-known line of AVM Fritz!Box routers.
A German Topic about that is here.
