You will want to use the D(digitstopluseindtmf) option on your dial cmd. 
That is a capital D for the option!
ex.
Dial(SIP/2100,D(1000))
-Jon
Stephen wrote:
> Hi All,
>
> Can Asterisk dial extension which resides in the PABX?
>
> (eg. 2000) Sip Phone <-----> Asterisk <------> ATA (FXS) 
<------>
> (CO side) PABX <-----> Extension (eg. 1000)
>                                                                         
> (2100 & 2101)
>
> can my sip phone call to pabx extension 1000? What will be my dial plan?
> I know I can connect to 1000 by dialing 2100 from sip phone after PABX 
> answer my call.
>
> But that's too troublesome, Can I just dial 21001000 instead ? which 
> mean the first 4 numbers are for pabx and the next 4 numbers are for 
> extension?
>
> Thanks,
> Stephen
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