Well, it seems the many requests for voice only calls in Duo have been answered. Playing around with Duo today I found a new toggle to use Voice Only or Video calling.
I must say, I am not sure why people want this feature when they are using a phone already, which is voice only calls, but it does seem like this was one of the most requested features we see in the Duo Product Forum.
Switching the toggle to voice only seems to just turn off the camera so there is no longer live video feed. When in Voice Only mode, you will only be able to place voice only calls to those that have the feature on their device, so if they are using an older version of Duo, they will be greyed out and you will see a message that they cannot use voice only with that contact at this time.
Its definitely an interesting additional feature that hopefully people will appreciate.