[sliken]

Trick to p2p is that generally you have to accept incoming connections for it to work. In signals case that's the signal servers.

Originally skype did this, skype users with a good network connection, good uptime, and who accepted incoming connections could self promote themselves to a supernode. This allowed async messaging for others, helped introduce peers who couldn't talk directly because of IP Masq/NAT etc.

So it's possible that signal could write a small application that could be a supernode. Ideally it could run on a Raspberry Pi, Plug computer, or even any of the numerous opensource routers. What way your battery sensitive phone wouldn't get run down by participating in a DHT or similar, but your raspberry pi could act like your inbox and facilitate incoming and outgoing messages.