try adjusting the PSTN line gain
check current setting with
config line 1 2 print
Default value for both RX and TX on PSTN lines is 0. Range is from -30 to +6
Increase your values with the commands (put new value in place of ‘X’)
config line 1 2 rxgain X
config line 1 2 txgain X
Increasing gain too much will introduce echo.