Cell phone service for Canada -> U.S. if no sim card?

When we visited, we just called Verizon and had them change our plan to add Canada and Mexico, then when we returned to the U.S. a week later we called and switched back.

If you'll be using multiple phones (in our case, 4) for some reason adding Canada and Mexico had to be done to each phone not just one time to your plan.

I don't know anything about sim cards, we all have iPhones with whatever they come with, we didn't do anything special to them.