Signal is probably safe, but if you don't mind if your messages get through third parties' hands while encrypted, you might as well use XMPP with OMEMO. XMPP is compatible with many kinds of devices, and it doesn't require a tracking device loaded with spyware to set up an account GNU Jami is even better: it is compatible with all major operating systems, you can create accounts anonymously, and your messages are not hosted in a central server, or any server: it's peer to peer, and message delivery only occurs when both parties are online, directly from one to the other, with end-to-end encryption I believe Tox and Briar are like that as well, but I'm not so sure