Jolla
#Sailfish OS Fiskarsinjoki update is now available to download for early access. It provides our users with new features such as conference calling and an option to save pictures and videos directly on your SD card. Read more about the changelog and the bug fixes at our blog:

Link: https://blog.jolla.com/early-access-sailfish-os-2-0-4-fiskarsinjoki/
9 comments show more
I think Signal integration will not happen, but Telegram and Riot.im (via libpurple) can find its way into telepathy-framework
vansid
There is an open source app based on Android that is still available in the Jolla Store. It's called LibreSignal Jolla Edition and it's written by leszek, the same author of the Webcat app.