What about GNU Jami? It's completely decentralized and can be activated without a phone number. It's a bit immature, but it works for the standard use cases. I

What about GNU Jami? It's completely decentralized and can be activated without a phone number. It's a bit immature, but it works for the standard use cases. In case Internet connectivity went down, you can can easily create a cheap message gateway with an Esp32 and Lora gateway. Another option is using of CB, but it's not encrypted.