How secure are Threema calls, and what’s the technology behind it?
Threema calls are as secure as Threema messages. Just like Threema messages, Threema calls are end-to-end encrypted. The call connection is established using Threema messages in the background; therefore, voice calls are just as trustworthy as chats, and the same verification levels apply.
Instead of a phone number, the Threema ID is used as unique identifier. This allows you to make end-to-end encrypted voice calls without disclosing your phone number.
You can learn more about Threema calls here.