I think the big issue with text messaging and offline recipients in ring is really that offline delivery tends to go with servers. With xmpp, or email, we expect the server to be online (up and on the Internet) all the time, and the user then connects to get messages.
There are crypto schemes to protect offline messages (OTR, OMEMO). But they don't protect against traffic analysis. Then there's bitmessage, which is probably ultimately challenged on scalability. I also played with Briar: https://briarproject.org/ which seems aligned philosophically with Ring. It uses tor hidden services (or local) and no servers, so ends up transferring only when connected. I found it painful to have a HS up all the time on a phone (battery use), so I wondered about having a Briar node of one's own in a stable computer and then connecting to it, sort of mixing the darknet and the client/server model. Ring could do this for messaging, where one has a proxy ring address that's likely reachable, and then the mobile client could connect and get things. But, that's probably going far afield of the original online voice call intent. It might not be hard, sort of MX records for chat destinations, signed by the original key, and fetched/delivered once and cached.
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