Some settings that you can try, the info is from the admin guide.
The following 2 are in the PSTN Line tab.
SPA To PSTN Gain - dB of digital gain (or attenuation if negative) to be applied
to the signal sent from the SPA to the PSTN side. The range is -15 to 12.
The default is 0.
PSTN To SPA Gain- dB of digital gain (or attenuation if negative) to be applied
to the signal sent from the PSTN side to the SPA. The range is -15 to 12.
The default is 0.
The following 2 are in the Regional tab.
FXS Port Input Gain - Input gain in dB, up to three decimal places. The range is
6.000 to -12.000.
The default is -3.
FXS Port Output Gain - Output gain in dB, up to three decimal places. The range is
6.000 to -12.000. The Call Progress Tones and DTMF playback level are not affected by the FXS Port Output
Gain parameter.
The default is -3.