I didn’t know what the effect was called, but after some searching I found this:
In telephony, sidetone is the effect of sound picked up by the telephone’s transmitter (mouthpiece) and instantly introduced at a low electronic signal level into the receiver (earpiece) of the same handset, a form of feedback.
And…
Almost all land-line (wired and wireless) phones have employed sidetone, so naturally it was an expected convention for cellular telephony but is not standard by any means. Usability experts believe that lack of adequate sidetone causes some people to shout or speak too loudly when using a cell phone (this behavior is sometimes referred to as “cell yell”).
Cell Yell…. LOL
RE Listening
I have been guilty of CELL YELL! Once on a train ride from Washington, DC, to New York, I got a call from a distressed friend who was yelling all on her own, and my voice got louder and louder, mostly trying to tell her I could not talk just then. When I finally finished the call, which went on for a good part of the ride, all the passengers in the train car burst into applause! Oh, the humiliation of it! I have since been more than careful not to yell on my cell!