If you're on Orange you might be able to get a "try it for free" package, I can't remember what it's called but they let you have about £5 worth of calls internationally (only if you've not done this with your phone before), did this last year, very handy.
Or else use your company phone & try to pass it off as expenses, it help if you work for a french company
Personally I'd just use walkie talkies once on site seeing as how cheap they are & good for pranks with other peoples conversations!!