Although the cheapest way to do this is with a phone card purchased from a Tobac in France, the easiest way is to buy an AT&T prepaid card from Sam's Club or Walmart.

From any coin phone in France, pick up the receiver, insert NOTHING, dial 0800-99-00-11 to get AT&T in the USA, dial the 800 number from the purchased card, enter the PIN, and dial just about any number in the world! Calls from France to the USA cost ~ US$0.21 per minute with this card. Calls from the USA to France cost ~ US$0.12 per minute.

Some hotels charge tons for access and others bar these calls from the room. Still others connect it for free (dial 0-0800-99-00-11 from your hotel room). I don't mind using the coin phones most of the time.

Yes, I know that there are cheaper and better solutions. But the AT&T card has ALWAYS worked well for me. And I leave home with it in my pocket, too.

Regarding your cell phone: You can purchase a prepaid SIM for your cell phone for ~US$20 in France. Your GSM (only) phone will have to be unlocked and use 900 and/or 1800 MHz for this to work. Then calls from anywhere can be RECEIVED for free. Note that callers from the USA pay a hefty per minute surcharge (my cheapo LD carrier charges ~US$0.34 per minute to French cell phones and US$0.06 to regular phones).

Enjoy France. It's a wonderful place!!