Why can’t I hear phone calls through my IPhone to my hearing aid?

They may well be iPhone compatible. And there may be some setting in their app which you need to turn on for them to do so, but they aren't made by or sold by Apple, so it's usually easiest to contact the hearing aid manufacturer for help with their product.

Check with your hearing aid manufacturer to see if it works for phone calls through your iPhone. Not all hearing aids do.