

On Android, there is Quicksy, and yes, it uses phone number sign-up for ease, but it seems more private than corporate apps.
Without phone numbers, there’s Conversations.im. There are some public XMPP servers with open registration, but you would need to find one, e.g. sure.im by Tigase lets you make aan account, but I don’t remember if calls work.
Are you looking to self-host something as well? Snikket is all-in-one chat+calls, but you would need to host a server. (Still based on Conversations and XMPP)











…made by developers with a special student/hobby account, only for a limited number of devices.
Half-assed “solution”.