The Marratech client has a powerful one-to-one function that can be used to send audio and text messages (Chat) to a single recipient.

The private window can be opened by right-clicking a user or using the Private 'P' button.

Talk to the other party by using the Phone button.

If you wish to mute the audio from the other participants, you can use Mute Public Audio (middle/third mouse click inside the private chat area, that's alt + mouse click on a Mac). This feature is useful for having a private conversation during an ongoing meeting.