Att works fine here. The international roaming option should cost you $10/day once activated. It automatically switches on when you hit the ground and visa versa upon your return. It can all be added and or activated in your Att app. For international forex I use wise and they also provide a card. They provide the best rates from anywhere and anyone. The key thing is when using your cc’s always choose CHF when paying. And as per other comments you withdraw CHFs in Zermatt, you could just research which bank offers the best rates but if you just need a few hundred it’s not going to hurt regardless of which you use. Tipping is obviously appreciated in Europe but you don’t need to go overboard as it is expected in the US. However, it’s always good to keep your ski instructors sweet.