We, the English-speakers, had a running joke about the cabines téléphoniques. Find the hottest shadeless spot in town, and you will find one. It was nearly impossible to read any of the displays because they were faded/melted or just the glare. I was good for about 30 seconds in the daytime before I was cooked. The phones operate on a telephone card, which you can buy in post offices and tobacco stores.
A cell phone was handy. We bought SIM chips for our unlocked cell phones, which we sold on eBay when we returned. When I called, any response except oui, non, and complet was beyond my language capability, so having a cell phone was very convenient for asking someone with language skills to help make a reservation. It did not require dragging them to a hot cabine téléphonique, or spending their money using their cell phone.
An unlocked iPhone may be the best technology at the moment. You can make calls, and use wifi when it is available.