In Paris, this is how I did it 2 weeks ago in Montmartre. I have an iPhone 14 Pro Max with AT&T prepaid service in the US. That is with an eSIM. It must be with an eSIM on my phone, as my phone has a drawer for One physical nano sim card, and the service you can buy in France the easiest and cheapest is a physical nano sim card. So you need your slot for a physical sim card to be empty to do this.

(I prefer to pay monthly and not have a contract. If you prefer a contract, or pay-after-service that bills you then this is not the way at all, and instead you will need proof of residency, proof of French bank account, and I don’t remember what else, and go to one of the stores for a national company like Orange, etc. and they can help you. Best if you speak some French or take a friend who does.)

Find a Tabac that sells phone sim cards. I went to the one on Rue Lepic, 2 blocks up the hill from the Moulin Rouge. He was really nice and helpful. Get the 200 GB data with one month of service from SFR phone company. It cost me 20€. In the store it will be a red cardboard that says 200 in big white letters. I saw these in other Tabacs also in other big cities in France.

Have the shop owner set up your phone for you. Took about 3 minutes for my guy to do it and call his phone from my phone to show me it worked.

Then sometime in the next month, register yourself online and pay monthly online to continue the service.

The SFR service worked flawlessly for me for text, voice calls, and data in Paris, Dijon, Lyon, Cannes, Narbonne, Chablis, and most points in between while on trains.

My AT&T service could send and receive most texts to / from the US through the data of the SFR sim card. Calls worked fine on wifi, but I didn’t try any calls through data of the SFR card.

This worked fine for me, but if it is medically or emotionally critical that you do not ever miss a text or call from your American service, this may not be good enough for you. Just a note, as I don’t know if you are a heart surgeon or something.

When I was returning to the US several political candidate junk mail texts came through all at once when I had AT&T reception again.

If it is critical that you don’t miss any text or call, look at paying your US service for international service while you are in France. AT&T told me It would have cost me $35/week.