RibbitRadio.org is slowly gaining popularity. It is a great way to send messages to people off-grid or with no cell or internet service. You only need a HAM or GMRS radio, and a smartphone (iOS or Android) device to get started, though there are some useful accessories that are nice to have. What makes this unique is it is open source, it works well, and I have been quite happy with the Ribbit code (It’s under Rattlegram on Android, and on iOS, Rattlegram requires Test Flight as it is in beta.) You can shut off cell and WiFi, but will require access to the microphone on the Android or iOS device in order to hear the digital packet information from the HAM or GMRS radio.
Open Rattlegram on your phone using the app, construct your message you want to send, make sure that you can communicate to the person you are sending it to via HAM or GMRS radio, then hit the PTT key, and hit TRANSMIT. You will hear a series of burst tones on your phone (speaker must be loud enough to send over the radio), and once the burst noise is complete, you can release the PTT key. The message will be sent to the recipient who has the Rattlegram app on the other end, as long as they have the radio volume turned up, and the Rattlegram app is open on their phone.
Communication range varies on terrain, equipment, and line of sight, but if all those are favorable, you can get quite a distance with these (I have been successfully able to communicate between two devices at over 1 mile with a ton of obstructions including a steel wall). With the right conditions, and a good antenna, I was able to get a few miles and the messages were sent and received.
This would be good enough to send a text message to someone during adventuring outings, but both the users will have to be on the app at the same time (background message receive is not available at this time.) A Meshtastic platform supports background messages, and also can alert your watch of incoming messages.
When I have used it in the field to test, we all agreed to test it on the hour so all of us had the app open at those times. That seemed to work well for check-ins and group messages (yes, if everyone is on the same channel/frequency and using the app, they all get the message.)
Try it out locally, and if you like it, take it on your next adventure. Updates to the platform will continue as far as I know.
Enjoy!