[cisco-voip] Intercom in Callmanager

[John Osmon]

On Fri, Oct 28, 2005 at 02:21:41PM -0500, Schuett, Court (MBNAP it) wrote:
> That's what we did.  We ran from an ATA 186 port into the amplifier that ran
> into the speakers.  We had a wiring company set up the amp and speakers for
> us.  It was pretty simple and it sounded like they had done it before.
> Connecting it to the ATA was also very simple.  Just assigned the ATA a
> number and when people want to announce over the speakers, they just call
> that number.  There was a catch to it though.  When the number was called,
> the speakers would pick up, but they wouldn't let go.  What happened was the
> person would call, make the announcement and then hang up.  But the speakers
> would still be broadcasting and would eventually go to a busy signal.  To
> fix this, you have to go in and change the polarity on the ATA to 0x00008 I
> believe.  Might want to call TAC and confirm that though.

Classic disconnect supervision problem.  When you have an analog line
it is difficult to tell something at the other end that the call is
over.  (Humans are easy -- the 'goodbye' protocol signals the end of
the conversation, so they hang up.)

Disconnect supervision on analog lines is typically handled by:
  - shorting the line (removing voltage)
  - reversing polarity of the line
  - playing special tones (rare)

Don't forget that the end device has to support that particular
disconnect supervision as well...

Last I'd heard is that the ATAs have *NO* disconnect supervision, but if
you've gotten them to break with the speakers, obviously *something*
is working.  Thanks for the tip.