3G/4G availability on the voie de Tours?

I'm trying to decide what to do about consolidating gadgets so I have less junk to carry. A smartphone is not an option for me because of the situation where I live in the US. I had planned to pick up a cheap prepaid phone in Paris and use an ipod touch for Iphigenie etc., but then I was wondering if I could just use VOIP on a cellular ipad mini and only carry one device. However, people I know who've spent a lot of time hiking in some of the more remote areas of France have warned me that 3G/4G is not widely available outside of the more developed regions. I can guess this might be a problem in les Landes, but does anyone have any insight about the rest of the route?

Thanks, GAUVINS.