Can a smartphone be of any use if the internet/cell service goes down?

Most messaging apps depend on having an active internet connection. All modern phones have WiFi mesh and Bluetooth/BLE hardware capabilities, but it seems there are very few apps that use them.

The existing apps have little overlap across platforms. Bridgeify is the only one that exists on both iOS and Android (although I didn't manage to make it work between different platforms).

I'm looking for an app that could help people communicate using their iOS/Android phones without internet connection by relaying messages peer-to-peer.

If it doesn't exist, any ideas why? How would you build one?