[Asterisk-Users] Outbound dialing issue with FXO

[Rich Adamson]

> >> However, outbound calls are hit or miss. Sometimes they work fine and 
> >> other times we get a "you must first dial a 1 or 0" message back from 
> >> telco when dialing out standard POTS lines.
> >
> >
> > Did you get this working yet?
> >
> >
> Yes, it does seem to be working fine now by adding the "w"s to the 
> dialstring.

The following is intended to provide a little bit of info as to why
the "w" is needed when dialing via some central offices (pstn fxo's).

Several telephone companies still have older electro-mechanical central
office switches. The majority of these older switches have a bank of
"dtmf receivers" that are shared across all pstn lines, and are only
attached to each pstn line during the initial few seconds of a pstn
call. (There might be 20 or 30 receivers for a central office switch
that supports 5,000 pstn lines.)

When asterisk seizes the pstn line (goes off-hook), dial tone is 
usually provided within a second or two. However, the dtmf receiver
may or may not be attached and ready to receive dtmf digits in that
short period of time. (If the central office switch is slightly
under-engneered, there could also be a shortage of dtmf receivers
that _could_ result in a receiver not being attached to the pstn
line within the first second or so.)

One or more "w" in the dial string causes asterisk to delay sending
the dtmf digits, compensating for the delayed attachment of the dtmf
receiver in those central offices.

In very very general terms, the delay is only seen when interfacing
with analog pstn lines. (Newer central office switches that support
isdn typically are not designed/engineered with this one-to-many
dtmf receiver arrangement.)