If you are doing one on one calls, I hear good things about Signal, though I don't believe it supports groups.

Right now the most reliable is Google Hangouts, which is kind of sad that I trust google more than Zoom in terms of security.

Also their is Jitsi, which seems to be relatively new and Open Source.

That said, I am admittedly not the most knowledgeable about all these things, so while it may be a good idea to look into them, definitely research them rather than just take my word on them.