An Australian research group has found a way to apply WiFi mesh networking onto the Android operating system, allowing phones to act as access points over radio waves to transmit voice calls as data.
The system, named Serval, can relay VoIP calls between phones using their WiFi networking. Individual phones can also act as relay points, and theoretically should be able to bridge together a phone in a remote area with no service to one with access to the cellular network, where the call can finally be relayed to its intended recipient.
The Android phones with Serval also seemed to be a decent substitute for walkie-talkies, as two phones can connect and exchange voice data without network access. The researchers are planning to make the software free and open-source, though no release date has been set.