I don’t really like Apple, but once in a while they do the right thing. This comes from the App store’s new labels on apps.
Signal just has “Contact info” under the “Data NOT linked to you” category. This is just the phone number + contact discovery.
UPDATE
There’s another post adding telegram here. This is what it looks like:
The signal back end isn’t open source, so the source for that is “trust me bro”. XMPP and matrix back end is fully open source and self-hostable.
They have to store phone numbers, its their primary identifier and routing system.
Its also a single server / cluster all hosted in the US so by definition isn’t secure.
Signal backend is open source.
Edit: you might be thinking of telegram.