discord and messenger are pretty bad when it comes to privacy, neither even bother end-to-end-encrypting calls.

Signal really is the best choice, but due to the phone number registration requirement, unless you're fine with the one-time purchase of some prepaid SIMs or something, that might get a bit annoying.

SimpleX is decent enough with calls for now (when they work), but connection times can be abysmal

This is where I would jump to Matrix / Element, but Element is currently in the middle of re-making their mobile Apps, including the entire calling feature 🥴