[VoIP] Early audio aka non-sup call progress info

[John R. Covert]

A little-known feature of Asterisk allows you to send early audio
on DID trunks delivered to you via those mechanisms where the
carrier allows you to do so.  This will usually be the case on
DID trunks delivered via E&M or T1 as well as by SIP and IAX.
It will never be the case for a standard C.O. trunk terminating
on a loop-start FXO port.

CAUTION: Providing useful information (other than call progress)
without first going off-hook is toll fraud, and if Asterisk users
start doing it when they shouldn't, then providers are going to
refuse to allow the audio to be passed back.

In general, early audio can only be sent from the called switch
back to the caller, not in the forward direction.  This is not
always the case, but you should plan for it being the case.

Before explaining how to do it in Asterisk, here are a couple
of examples of what to do and not to do.

It's legit to provide a ring tone, busy tone, strange call
progress tone, vacant number recording, referral to the main
number on vacant extension, or similar response using early audio.

It's not legit to provide interesting historical recordings, access
to voicemail, or messages like "John is out of the office and will
return at 7:45 tomorrow morning."

Here's an example, which you can hear on CNET 1 263-5999
exten => s,1,Progress()
exten => s,n,Playtones(ring)
exten => s,n,Wait(2)
exten => s,n,Playback(holdline,noanswer)
exten => s,n,Playtones(440/45,0/45)
exten => s,n,Wait(1.4)
exten => s,n,StopPlaytones
exten => s,n,Playtones(442/47,0/47)
exten => s,n,Wait(1.4)
exten => s,n,StopPlaytones
exten => s,n,Playtones(413+438/400,0/200,413+438/400,0/2000)
exten => s,n,Answer()
exten => s,n,Playback(linux&lyrics-louie-louie)
exten => s,n,Hangup

/john