A user wrote:
Recently I went to Portugal and when I was there I bought a sim card and put it in my iPhone 11 Pro, with my primary e-sim still in it.
When I got home to Australia, I deactivated the Portugal sim-card, deleted it and took the sim card out.
Now when someone call me I respond with text, my iPhone changes the calling numbers country code to +31.
I had the same problem. Despite turning "Dial assist" on/off, checking Country, Language and all the suggested stuff it did not work for me.
What I did at the end was deleting country prefix from contact numbers --> Save --> Added country prefix again. It works now.