[Asterisk-Users] Outbound dialing issue with FXO

[Mike Clark]

Rich Adamson wrote:

>>>>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.)
>  
>
This makes perfect sense now as the systems we are installing are in 
small town/rural NC where there is very likely to be some older equipment.