Iphone 8 plus No wifi calling in list

My wife has a Iphone 8 plus. We are out of the country and want to turn on wifi calling, but there is no selection for wifi calling in the phone. We were in the same country last year and she had a iPhone 6 plus and we were able to use wifi calling. I have a android phone and have it connected to wifi calling now as I type, so I know that the carrier supports wifi calling. The question is: does the iPhone not have this feature build in, which I think is possible because other people have the same problem and have stated this on the internet. The current software version on the phone is 15.1.