The working mode of initiating private call by PPT is half-duplex communication. In half-duplex communication, data can be transmitted in one direction, but not in both directions at the same time. It means that in a private call, one party can press the PPT key to initiate a call and send a voice message, and the other party can only reply after receiving the message. Similar to the working principle of walkie-talkie, only one person can talk, and the other person needs to wait for the other person to finish before responding. This method can ensure the privacy of communication, but there will be a certain time delay.