Hearing back your own voice during call is known as VOIP (Voice over Internet Protocol).These are caused by several reasons like sound transmission bouncing back from the recipient's phone or by the sound on the recipient's phone being too loud, causing the received audio to enter the microphone on the mouthpiece of the recipient's phone, or caused due to network hops,unbalanced line or may also be caused due to hardware error like twisted wire connection between mic and ear piece,water damage,etc.
