The technology is based on the concept of 'cognitive digital radio' that enables users to turn their smartphones into peer-to-peer, off-grid communication tools.
The network generates its own signal through proprietary adhoc networking protocols and automatically coordinates with other units within range which enables users to send and receive text messages, share their GPS locations on offline maps regardless of access to WiFi or cellular service.
The Calculator app is loaded on a normal Android phone and communicates over WiFi to the Radio which further gets transmitted on VHF as digital burst communication.
The principle is same as Gotennna but with little difference.
Gotennna uses Bluetooth to communicate with the a small Radio device Antenna which further uses UHF to transmit data.
Gotennna works on P2P concept while this most likely is not working on P2P.
Terrorist are also using morse code and DTMF to evade interception.