You've probably already been and gone, but in case you haven't gone yet, when my husband and I went to southern France a few years back, we brought simple unlocked phones (bought off Amazon) and bought SIM cards when we got there (we were with Verizon and our phones wouldn't work there). We weren't looking to have a smart phone or data plan, though, so that option was pretty cheap.
Can't recommend shops since it's been many years since I went to Paris, but my recommendation is to bring your very best spoken French and use it as much as possible, even if it makes you nervous. It starts you off on the right foot if you try to speak French first thing when you meet someone. Of course, my French is so appalling that whoever I talk to answers in English (I think it may actually be painful for them to hear me speak French) which doesn't help me improve my French much, but they do seem to appreciate that I made the effort. I've heard people say that the French refuse to speak English to tourists, but I've never had that happen unless they actually don't speak English (I have had people claim not to speak English but then recant after I try to speak French to them--yes, my French really is that bad).
