Yes, but its tricky.
You need to find the wav file, recreate it as silent and save it back to the wav file directly.
I think the file is MC00.wav located in Voicemail Pro\VM\WAVS\enu
It's been a long time, so you may need Hunt for it. I've done this years ago to create a panic button using Voicemail Pro.
user pressed the button when in distress, and auto call a group in security so they can listen in on what's going on in the room the caller is in.
I had to rename a couple files so they were silent and did not tip off that a panic button was pressed.