extensions.conf

[default]
exten => _[0-9].,1,NoOp(#### [default] ####)
exten => _[0-9].,n,NoOp(we shouldn't ever be here, drop the call)
exten => _[0-9].,n,Hangup()

exten => i,1,Hangup()
exten => h,1,Hangup()
exten => t,1,Hangup()

[from-internal]
; internal calls between extensions
exten => 201,1,Dial(SIP/201)
exten => 201,n,Hangup()

exten => 202,1,Dial(SIP/202)
exten => 202,n,Hangup()

; send all other numbers to outbound trunk
exten => _[0-9].,1,NoOp(#### [from-internal] ####)
exten => _[0-9].,n,Set(CALLERID(num)=442036427900)
exten => _[0-9].,n,Dial(SIP/+${EXTEN}@trunk)
exten => _[0-9].,n,Hangup()

[from-trunk]
; send all incoming calls from trunk DDI numbers to extension 201
exten => _[0-9].,1,NoOp(#### [from-trunk] ####)
exten => _[0-9].,n,Dial(SIP/201)
exten => _[0-9].,n,Hangup()