If you care about privacy you can consider using i2p network and fork of Conversations, links below.
You don’t need any phone number to register a UUID.
There are more advantages to Signal but one is worth to highlight here.
Using xmpp + omemo (Conversations) you can get all messages on all your mobile devices. Signal/Molly allows you to have only one mobile device registered.
You can set up an account here: xmpp.ilita.i2p
The best solution is to set up own xmpp server. It is super easy if you have capability and money resources to buy Pixel and install GOS.
There is a ton of tutorials in the internet to do that.