Using mobile phones in France
How to use a cell phone (in French: "téléphone portable") in France, whether using your own, or buying or renting one to use while you travel
The easiest solution for international travel with a téléphone portable—or simply portable (cell phone)—is just to sign up for your carrier's international roaming plan and take your own phone—but that can be quite costly, on the order of $30–$45 per month, plus you still pay $1–$2 per minute to make calls and 50¢ for texts (beyond the token 100 minutes, 100 texts, 100 MB that comes with the plan).
Far better: Take your phone, put it into airplane mode, turn on WiFi, and use hotspots (at the hotel or in public places) to log in and use Skype to make calls for free.
The big exception: T-Mobile, which offers plans from $10–$20 that that works in most major countries and covers unlimited calls, texts, and data (well, they throttle the speed if you manage to burn through the included GB, but it still works).
I actually keep a cheap tablet computer just for this purpose, and sign up for a month of T-Mobile whenever I need it, using the data feature and Skype to communicate. (I then cancel the plan after I return home, because T-Mobile lets you do that.)
The other main options are to rent an international mobile phone (cheaper, in the long run, than most home carriers' international plans; just set your own number to forward to your temporary one), buy an international cell phone before you leave (if you plan to do a lot of international travel), or buy one when you arrive in France (easier than you'd think, and a good choice for longer stays).
Here are more details: