Cell phones are two-way radios, but they can't communicate without infrastructure (with very limited exceptions). To use Zello you either need to have 1) functional cell towers connected to the world by your provider's infrastructure or 2) functional Wi-Fi connected to the world by your provider's infrastructure.

The exceptions can send and receive data over short distances, so don't help much for this sort of scenario. Bluetooth has a theoretical range of 100 meters, but the class of Bluetooth generally present in cell phones is limited to 10 meters, so only good for maybe communicating within a modest-sized house. NFC is used for credit transactions and such and the devices must be rather near to touching, so also is not useful for long-distance communications.