[incoming-extern]
; Incoming call routed to local pbx
exten => _${LOCALFULLBASE}.,1,Goto(incoming,${EXTEN:${LEN(${LOCALFULLBASE})}},1)

; Incoming call with foreign DID to be routed by custom context [routes]
exten => _X.,1,Goto(routes,${EXTEN},1)

[incoming]
include => intern