Briar. It was designed just for this. All traffic routes through Tor, it can also send messages using wifi, data, or bluetooth. There are no central servers to hack, seize, or takedown. Pretty much the entire internet would need to be killed off.
As for face-to-face communications - that’s been done for centuries to fight the good fight. No reason that can’t continue.
I really don’t use the phone unless I go out. It has my shopping list, I scan my worksheet, listen to music, messaging, a phone call once in a while.