Mr. algazi returns a phone call to his client, who has left a voice message.

Feedback is returned to the sender.

Explanation:

In the channel of communication, there is a flow of information from the sender to the receiver. Usually, the sender has an idea or message which he wishes to pass to the receiver. Most times he uses a medium to pass the message. This medium could be paper or electronic devices such as is the case with Mr. Algazi and the messenger.  

Probably the messenger told Mr. Algazi to call when he is available.  When Mr. Algazi returns the phone call, he provides feedback to the messenger.

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