1 side audio when received call

[josiahteo22]

I am using a lower end model phone Yeastar T21

I configure it using IPsec, configure dyndns in my phone to connect back to my HQ mpbx panel.

I have no problem hearing when customer call me or intercom from my colleague from HQ or other branch.

The only issue now is whenever i make a CALL OUT, my customer can hear my voice but i couldnt hear theirs. Any solution to solve this problem?

I have tried to change a modem and still the same.

Appreciate help on this matter.

Teo