The only provider I could put on the list would be Signal.
The goal of the list is to list services, that even someone's Grandma, with no technical knowledge, could use. So I'm not listing self-hosted since it is not something everyone could set up, plus not everyone could even if they wanted to because of their local ISP.
If you could think of alternatives, I am always willing to explore them, and expand my list.
@Linux
#Quicksy is an app (iOS and Android) and an XMPP provider, both free.
You get an XMPP address of the form phonenumber@quicksy.im after SMS confirmation. This gets you automatic contact discovery from your address book, and you're part of the federated XMPP network (ie, you can chat with XMPP self-hosters).
I personally don't want my phone number anywhere, but Grandmas may want it, for a user experience similar to WhatsApp, Signal, etc.