Recently updated Fairphone 3+ from e.OS 2.3 to 2.4.
Not sure if the update was the issue but in order to temporarily ensure I could receive phone calls I had to temporarily use my old 1+ 3T. Last night (early hours of this morning), I delved into provider settings.
Voice calls were set to 4G. Dropping to 3G has solved the issue. Other comment is that my right-angled triangle for network coverage still has an exclamation mark over it. (GiffGaff).
Regain your privacy! Adopt /e/OS the deGoogled mobile OS and online services