[Asterisk-Users] PABX and Asterisk Dial Plan

[Stephen]

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