Test 1:
- Make a test phone call.
While on the call,
pull out the 4-core flat phone cable (between the base of the
phone and the USB recording box).
After a while, connect it again.
Continue the conversation
End the call.
When listning to the call, check if the noise went away,
when you pulled out the cable and before you plug it in again.
If so it is a problem on the PC, power dissipation. It can
be a transformer or the power supply.
Else it is a power problem between the PC and the PABX unit.