If you can locate it at a mesh node that has Internet backhaul, you can throw up a raspberry Pi running asterisk and just get a DID trunk line. From there you can either call straight across into that asterisk from voip phones on the network, or link it via an IAX trunk or something to another asterisk board across town.
DID lines run about $0.99 a month and a half-cent a minute for domestic calls. can put plenty of restrictions and such on the dial plan on asterisk too