[asterisk-users] Understanding Call Handling In Asterisk

[Trevor Hammonds]

On May 29, 2009, Varun Rapelly <varun.rapelly at spectross.com> wrote:

>  Hi,
>
> I am a newbie to Asterisk; need help understanding three-way conferencing &
>
> call-transfer features implemented over standard extensions i.e. on a
> TDM800P card (4 FXO + 4FXS)
>
> In Asterisk I have observed that if an extension is already participating
> in
> an active call (e.g. Ext A & Ext B communicating):
>
> 1. An incoming call to one of these active extensions would be presented
> with call-wait beeps (e.g. Ext A receives call-wait beeps as Ext C is
> attempting to call Ext A).
>
> 2. The call waiting may be answered by pressing Hook-Flash, placing the
> previously active call on hold (e.g. C answered; A & C communicate; B
> placed
> on hold).
>
> 3. The calls could be toggled by subsequent Hook-Flash's (e.g. A & B
> communicate; C placed on hold).
>

Yes, this is normal behaviour on pretty much every analogue PBX or telco
switch.


> Queries:
> 1. If the extension which received call-wait beeps hangs-up then the call
> waiting/the call placed on hold returns as a new call. I was expecting the
> call to be transferred (A hangs-up, B & C communicate), how could the call
> be transferred? I expected this feature to be available in Asterisk as this
>
> is a very normal feature available on any PBX and used extensively in Call
> Transfer.
>

When you transfer a call, the person initiating the transfer has to be
MAKING a call.  Example:  Ext A receives a call from Ext B.  Ext A wants to
transfer the call to Ext C.  Ext A puts the first call on hold with a hook
flash, dials Ext C, then either waits for the Ext C to answer and announces
the transfer (e.g. an attended transfer) OR simply hangs up as soon as the
call to Ext C starts ringing (e.g. an un-attended or blind transfer).

The behaviour you explain is not something available on any switch that I am
aware of, and would be highly problematic if it were.  If this "feature"
were available, you could get a circumstance where two people who are
calling you end up being bridged together on a call, unknown to you.  As a
bad example, your wife and your girlfriend end up talking to each other
because you hung up while one of them call-waited you while you were talking
to the other.


> 2. How could the extension that received call-wait beeps initiate a
> three-way conference with the other extensions (A, B & C in three-way
> conference)? I expected this feature to be available in Asterisk as this is
>
> a very normal feature available on any PBX and used extensively in 3-way
> Call Conference.
>

Again, this is NOT a feature available on any analogue PBX that I am aware
of.  If it were, this would, again, mean that you may get unwanted parties
connected together.  With the above example, you answer your girlfriend's
call while talking to your wife, and all three of you end up in the same
conference.

Unfortunately, POTS lines do not handle transfering multiple inbound calls
very well (with call waiting).  This is not an Asterisk issue, POTS lines
were not designed to do anything other than handle a single call at a time.
You may be able to handle transfering a call-waited call with DTMF
signalling.  I am certain someone else on the list will be able to give you
a definitive answer on that.


Sincerely,
Trevor Hammonds
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