International roaming charges will be always ridiculously high. So the best option for you is to unlock your iPhone and use it with a local SIM card from France. This would save your phone bill by a huge margin. You can unlock an iPhone using remote unlocking service. You can get the remote unlocking service for an iPhone from ProUnlocking.com This method of unlocking an iPhone is safe, simple, affordable, quick and permanent. It doesn't require a jailbreak too. Good luck.