Non licensed off grid texting… think of CB radio only text based that uses each radio as a repeater as well so if you get a few high enough that their radio waves are not disrupted, you can reach across states 

 So, like, decentralized pagers 

 I guess, I wasn’t cool enough to have a pager… even though I tried like hell to win one of them Mt. Dew pagers back in the day 

 Is it a US only thing? 

 No I’ve seen people in the UK posting about them also