Outside of Signal, nostr:nprofile1qqsvnx99ww0sfall7gpv2jtz4ftc9v6wevgdd7g4hh7awkpfvwlezugpz4mhxue69uhhyetvv9ujumn0wd68ytnzvuhsg5cway is fantastic if you don't want a centralised messenger or a phone number requirement. 

 SimpleX and Session are the best.