Replace the SIM card with a French one? You may need to get your phone unlocked as well. Although most models these days are dual or tri-band compatible(Vodafone and O� are 900gsm, Orange and T-Mobile 1800gsm), you may need to check your handset's specification first. Try doing a web search on your handset's model name and whatever French networks you find.
I've got a Spanish vodafone SIM card, all you have to do is to go in a vodafone shop in the country where you are and they'll sell you one. Mine in Spain cost 24� and I got 18� of FREE call time too. If your phone needs unlocked then check out http://www.uniquephones.com/unlock/ and you can do it there FREE.