extensions.conf:

[us-it]
include => internal
include => local-trunks
include => international

[finance]
include => internal

[internal]
include=> local-extensions

exten => _6XXX,1,Dial(SIP/${EXTEN}@someSIP,20)
exten => _5XXX,1,Dial(SIP/${EXTEN}@someOtherSIP,20)

[local-trunks]
exten => _9NXXXXXX,1,Dial(Zap/g1/${EXTEN:1})

[local-extensions]
;you could split this section up more into local-extensions-finance, local-extensions-us-it etc
;but i'll lave that as an exercise for you
exten => 8234,1,Dial(SIP/cartman)
exten => 8235,1,Dial(SIP/tweak)
exten => 8236,1,Dial(SIP/butters)
exten => 8237,1,Dial(SIP/chef)

[international]
exten => _1011X.,1,Dial(SIP/${EXTEN}@someSIPwithInterntational,20)

[sip-in] 
;; change this to whatever context your sip peers which are sip connections
;; from another box
;; if its asterisk however, i would recomend using iax.
include=> local-extensions