How to let asterisk call an extension under normal PBX?

hello all, i have an SIP device conect to normal PBX, for example dial 1000 will enter the PBX through the SIP device. there are many extensions under the PBX . how to let asterisk call these extensions using call files?

Channel:SIP/1000 Callerid:1000 MaxRetries:0 RetryTime:10 WaitTime:60 context:internet-call extension:1001 priority:1
in this example 1001 is another sip client. when put this call file under /var/spool/asterisk/outgoing 1001 will ring, when answer,you will hear the prompt voice from the PBX and indicate you to input the extension number. then conversation begin.
i want to ask is how to pass extension number as an parameter in the call file ? thanks in advance!