In my experience, when you add a MWI button on a phone, when it's lit and you press it, it rings to the mbox of the MWI button.
But only when it's lit.
Try this: when the button is lit, pick up the handset to get dial tone from any other key than your prime line and then press the button. How many times does it ring before VM answers?
If it rings ~4 times that would suggest to me that 4122 lit the light.
Ralph