[asterisk-users] 2B Channel Transfer on XO-based T1

[Jared Smith]

On Tue, 2009-04-14 at 17:52 -0400, Max Metral wrote:
> I’m trying to get “blind transfer” from an incoming DAHDI line to an
> external number to work on an * 1.6 install using a T1 from XO.  The
> documentation is very “distributed” and incomplete, so while it’s not
> working, it’s definitely more likely my error somehow.  Couple
> questions if anybody is out there who even knows what TBCT is…

> 1)     Is this even supported?
s
Yes, it's supported in Asterisk and DAHDI, but your success in getting
it to work will depend on many factors.  As I understand it, it only
works with certain switch types (I've had the best luck on 5ESS), and
only when the telco enables that feature on your trunk group.

In my experience, the telcos usually don't enable this feature by
default, and it can be a pain to talk them into enabling it.  

> 2)     Does it require some settings in dahdi_channels, or features,
> or whatever?

It requires the following features be enabled in chan_dahdi.conf (or
zapata.conf, for later version of Zaptel):

facilityenable=yes
transfer=yes

> 
> 3)     Would I “trigger” it via a Dial command or commands, or via
> Transfer?

Neither... it happens automagically!  Some time after the second leg of
the call has answered, Asterisk will send a facility message to the CO
switch saying "Hey, mind bridging these two calls on your end, so I can
free up the channels on my end?"  If the switch says "OK", you'll see
the calls disappear from Asterisk (and the people on the calls won't
know the difference).  Otherwise, the calls will continue to be bridged
by Asterisk.

Obviously there are options to the Dial() application that would
preclude Asterisk from allowing the transfer to happen, such as the t,
T, w, and W options (and I'm sure there are probably more).

> 4)     Do either or both of the legs need to be answered?

It's my understanding that both legs need to be answered and bridged
before this will happen, but I'm not 100% sure.

One other minor thing I'll point out... assuming that your 2-B-channel
transfer is successful, the telco will send a message to Asterisk at the
time the call is eventually hung up.  Unfortunately, Asterisk has long
since forgotten about the call by that point, so it simply writes a
harmless warning message to the console and goes on its merry way.  (If
a developer happens to read this and needs a pet project -- it would be
nice if this would update the CDR records for the original call!)

I hope that's enough documentation to get you started!  Please let us
know how it works out for you!


-- 
Jared Smith
Training Manager
Digium, Inc.