Hi all, I would like to know how can I immunize the background noise in my case. Anyone can help? I have adjusted txgain & rxgain in different value but the result is the same. ango Below is my configuration. asterisk1.4.21.1 zaptel1.4.11 addon1.4.7 TDM400 (FXOx4) There is a very large background noise when a call from sip to PSTN. Below is the test case. A - sip phone B - PSTN case 1: result is normal caller A1 callee A2 case 2: A hears B is ok but A hears much background noise from his/her area when A say nothing. The sound volume of A is very low when B hears A. caller A callee B case 3: same as case 2 caller B callee A