A different kind of security problem?

["Smith, Keenan C." <Keenan.Smith () jhuapl edu>]

All,

Okay, this is one that requires squinting your eyes a bit and turning
your head to really be a security issue but if you stick with me, I
think you'll see where I'm going with this one.

A recently installed VOIP system in my company works well most of the
time.  However in a few cases, we've heard music on the line that was
not being generated by either end.

In one case, one of our VOIP phones was being used to access a bridge
for a conference call.  There were 15 or 20 folks on the call and at one
point during the call loud rock music begin playing.  The phone was hung
up since voices could not be heard over the music.  We found out later
that not only had everyone else on the call hear the music but it ended
when we disconnected from the call.

In another case, one of our VOIP phones was being used to call a
residential number.  Again, towards the end of the call music begin
playing.  However, this time it was soft jazz.

In both cases the music was not being generated by any of the
participants and none were on hold so hold music was not the culprit.

My thought is that we're experiencing a bleed through or cross-talk from
other traffic on the same network hosting the VOIP traffic.  

If that is indeed the case, my question to the group at large is this:
does that indicate a possible vulnerability of some sort in the
protocols that support VOIP?  Is this something that could be exploited
to eavesdrop or even route a call to an unknown destination?

I Googled for this but didn't find much about it.

Thoughts?

Keenan Smith