If encryption works for text-based chat, voice seems like a natural extension. If only it were that easy.
Encrypted voice calls can circumvent government wiretaps, or criminal snooping. But a host of technical challenges with facilitating the calls themselves has slowed the spread of voice over internet protocol overall.
You can read the details in an article by Lily Hay Newman in Wired at http://bit.ly/2p566TT.
However, encrypted voice applications do exist. See https://duckduckgo.com/?q=site%3Aprivacyblog.com+voice+encryption&t=ha&ia=web for some past articles about voice encryption, mostly about applications that are available today.