Instant Messaging is such a walled garden, actually. o.o
There are no real "multi messengers" anymore like Meebo and IM+ back in the day. There is Beeper, but it runs on Matrix and I would consider it commercial; and it uses Synapse... x.x XMPP gateways seem to be far and few out there - I found a few very old ones for Telegram but none for Signal.
Man, are people really running a multitude of IM apps on their phones these days now? Is that actually a thing? O.o
Guess I am stuck with my Matrix messenger for a while... Well, I could work to implement this in Nostr, I guess - but there are a few things I would have to work out first. Bridges rely on controlling your account; and not everyone would trust a remote service... so I would have to figure something out. o.o