Other than Signal, I also recommend Threema. It doesn't rely on mobile numbers, possible to configure to run on your private server, etc. It's just not free (as in beer). Also, it's from Switzerland, a country respects your privacy more than the USA[0].
Well gouv.ch might, but Crypto AG was an NSA front for decades so I wouldn't be so certain about the companies.
If I wanted to lure people in on the pretence of security and privacy, Being Swiss would be good bait.