echoes during intercourse iPhone 4S

During the communication with each subscriber of my voice is a mirror in my same dynamics. So, what I say - immediately hear. If you speak up - and then over time I will be heard louder. At first thought the cost of cellular networks and / or phone on the other side. An no, it's every subscriber is a problem. Yet. Sound notification that is triggered during my conversation with the caller works very strange. I hear a distinct echo of sound, a feeling that the machine is wrong, hollow space. If you attach a dense screen phone to your ear or the back of the rattling and finger on the case - heard a dull long, slightly elongated sound. Thus, I see there is a problem that makes itself felt. And clearly shows that the problem is hardware. What can you say about this issue?