I understand France has the deepest penetration of wifi in the world. So, why get a cell phone in France?
Of course you can make Wifi phone calls in France, but you will not be reachable at all time. The Wifi covering is good, but there are lots of places (outside of town or in hilly landscapes) where the connection is harder to catch. Remember, 95% of the Wifi signals are protected by a password. So you'll need to use the network of a friend, of your hotel, B&B, restaurant or café. Unless you use a public hotspot (different rates according to the local Telephone Company). Note that there are almost no public (coin-)phones left, now that everyone is equipped with cell phones...
Have a nice trip to France,