No. The PBX audio is set at line level. This means that raising it any higher will introduce distortion to the audio.

You will need to find another method of amplifying the audio signal before it reaches the Vattendance PBX.

Suggestion:
Purchase a Behringer MX400 and place between your Mixer and PBX.
This will provide you with complete volume control.
In New Zealand you can find it here.
In Australia you can find it here.