There already exists some P2P mesh phone networks TerraNet[1], serval[2] and FireChat are a few. None of them involve payment to phones you use as relays. Still,  that doesn't mean the idea of paying is dead.

My latest thinking is to have wallet numbers as phone numbers. That way you can be sure who is calling you and that you have got through to the number you dialed (various ways of doing that). Still not sure if relay phones can check in real time if they will get paid (by looking on the blockchain to see if the wallet has enough coins in it) but I don't think being cheated would be a big problem in general. Sure, someone could write an app to cheat the system by generating wallet numbers that it uses one time (or a few times)  with no coin in it but they'd lose the ability to identify them selves to the callee, hardly seems worth the effort, if fees are reasonable. I'm not into hardware of phones so don't know how many models have wi-fi (which seems a good way to form the mesh). If wi-fi is common (and accessible from an app) then if apps are written for a few platforms perhaps the system might boot it self and grow slowly, in different geographical areas to start with. The phone can use the conventional centralised system if no mesh phones are near by. If I knew much about writing such apps I might of had a go. As it  I write C++ CAD/CAM code and am getting on a bit, so am not sure if I could make a decent job or even get the job done before I snuff it. So I'll leave it at that for now.

[1] http://www.terra.net.lb/
[2] http://www.servalproject.org/