Optimal, no mobile number or email required, works over TOR
CONS:
They monetize with a token (altcoin) by the name OXEN, it is their own network and the nodes of the privacy token carry the data end to end encrypted, the liquidity is poor and therefore a potential 51% attack is plausible, although not likely.
Optimal voice/video quality, no mobile or email required, novelty app, decentralized
CONS:
A battery killer, during configuration, to avoid killing your mobile battery set the notification to work only when the application is open. This setting will render the application useless for receiving urgent messages or calls, you will need to agree with the other party to use the app. It is quite new, here an article I wrote recently about it4
Footnotes