I had this type of issue with iOS 12. It turned out to be some kind of network provider issue as my phone always thought incoming calls that started 0044 were still here in Switzerland. Changing absolutely nothing else, I swapped sims with a different provider and then the calls were correctly recognised as coming from the UK. I had (and still have) dial assist turned on.
Odd, huh. I never got to the bottom of it, as the reason I changed providers was due to better rates rather than any attempt to fix this issue, and I've not moved back.
Odd, huh. I never got to the bottom of it, as the reason I changed providers was due to better rates rather than any attempt to fix this issue, and I've not moved back.