At times you may experience an echo on your line which is often only heard by one of the users.
Problem
The sound from the microphone in the mouthpiece is circling back into the phone's earpiece causing the caller to hear an echo of his voice.
Cause
This is often a problem caused by a time lag. The echo can also be caused if the recipient's phone is too loud, causing the received audio to enter the microphone on the mouthpiece of the recipient's phone.