Voicemail low messages

My config: Sangoma appliance, ip phones: sangoma s500 et polycom vx400
vega50 gateway for one BRI line, an sangoma FXO card 4 ports, 2 analogic lines.
Cisco SG500-28P switch

When I do :
*97 / 0 / record my unavailable message on sangoma or polycom ipphones
2 to listem to my recording sound is OK.
then call from outside , wait for the voicemail system to play my unavailable message is barely audible, but the system message like ‘good bye’ are OK low but OK.

I have others audio problems with the analogic lines but not the BRI lines. I don’t know if there could a relation with the voicemail system audio problem.

The office was closed, pc off . Except mine of course.

Where should I go to improve that?
Many thanks