OPPO has announced a new technology at MWC Shanghai: MeshTalk. It is described as a way to talk or text without the need for a cellular, Bluetooth or online connection. MeshTalk apparently works when 2 compatible devices are within a maximum of 1.86 miles of each other.
https://www.notebookcheck.net/OPPO-premieres-a-new-kind-of-connectivity-independent-of-Bluetooth-Wi-Fi-or-cell-data.426839.0.html