Putting the above aside: If Tox doesn't work for you, your best mobile-friendly options are: 1) Signal and 2) as others have mentioned Matrix - if you want a self hosted and federated solution.
[edited: addendum] i now realize you probably mean p2p-encrypted messaging. Damn semantics of non-formal languages!
2. Tox - p2p
3. Whatsapp - Not p2p
4. Signal - Not p2p
Putting the above aside: If Tox doesn't work for you, your best mobile-friendly options are: 1) Signal and 2) as others have mentioned Matrix - if you want a self hosted and federated solution.
[edited: addendum] i now realize you probably mean p2p-encrypted messaging. Damn semantics of non-formal languages!