yeah, in southeastern europe it was viber, i think northeastern europe it was telegram, it's whatsapp here in portugal, my landlords and ladies all have asked to use it, even though we all have $10/month SMS prepaid accounts with vodafone and whatnot

signal was actually the first, they merged out of redphone and some other thing i forget, some SMS type deal, they actually were the ones who pionered using SMS as auth right in the beginning, circa 2013, i had redphone on my sony xperia android back then

so, yeah, you obviously were not there when it was all starting, circa 2012-2013, it was definitely signal first, and everyone else was riding on top of that, and it was open source originally, that's part of how it happened

signal -> whatsapp -> telegram is the sequence i recall

i was in balkan so maybe it looked a little different, they had viber, which also was big i think in west asia, ie india, pakistan, georgia, etc