Settings on your phone - network , restart the phone, you can replace the SIM card and replace the local test. The fault persists , then open the phone priority , select the network mode to use only 2G networks to test recommend that you re- install the SIM card. Or else when installed microphones in the connection port that is not connected , you can reconnect it.
Only show your touch screen is good , it is estimated the problem is the screen above . Touch screen with display is not the same as two touch screen just above a transparent display panel only. you can buy a new lcd. I recommend DBXStore.
It depends on the ROM. Most newer 4.2 ROMs have already fixed the issue but try turning on Noise Suppression within the dialer's settings. Go to phone, click the menu button, then navigate in settings and scroll down until 'Noise Suppression' shows up. Tick the box to check mark it and the echo should be gone. Noise suppression also helps with complaints about muffled sound.