Session is generally pretty safe, they use their own encryption which is the Session Protocol and all messages are onion routed and end to end encrypted.
You also don't need a phone number or anything else when signing up for it and because of the onion routing your real IP isn't even exposed and there's no central server from which Session could read all messages etc.
qTox is also a option and you don't need any phone number or similar to sign up but it does leak your IP address unless you use a VPN/Proxy. You can also run qTox through Tor but that only hides your actual IP and is not comparable to what Session does with onion routing. qTox also doesn't support offline messaging and the qTox as far as I know has not been worked on since 2022.
I think /threads/133659/ goes into more detail on that and some other things about Tox
Сессион и симплекс хороши только моментальным регом и встроенным тором. Оксэн (разрабы) вроде-бы целый децентрализированный и2п нетворк (локинет) делают для интеграции, вряд-ли это многовековой коварный план чтобы хлопнуть сразу всех. Аном был с закрытым кодом, на этом можно и закончить.