Other than the Briar Project, I know of nothing that addresses handset to handset direct comms. And no one I know is even vaguely interested in it and few, other than activisty tech types, even think the idea has value.
It's maddening. castoff phones on thrift stores, wifi only even, no sim, have more computing power than most people had in 1990. And no interest in them except as commodity platforms for whatever it is we do.