Dialplan:

[internal]

exten => 666,1,Dial(SIP/666,30,tTW)
exten => 666,n,VoiceMail(666@imapvoicemail,u)
exten => 666,n,Hangup()

exten => #123,1,Dial(Local/666@internal/n)

exten => ####,1,Dial(Local/666@internal/n)

exten => 800,1,Answer()
exten => 800,n,Playtones(ring)
exten => 800,n,Wait(10)
exten => 800,n,StopPlaytones()
exten => 800,n,Hangup()

exten => 101,1,Answer()
exten => 101,n,Noop(${QUEUE_MEMBER(CurlyQueue,count)})
exten => 101,n,Noop(${QUEUE_MEMBER(CurlyQueue,logged)})
exten => 101,n,Noop(${QUEUE_MEMBER(CurlyQueue,free)})
exten => 101,n,Noop(${QUEUE_MEMBER()})
exten => 101,n,Noop(${QUEUE_MEMBER})
exten => 101,n,Noop(${QUEUE_MEMBER(CurlyQueue)})
exten => 101,n,Noop(${QUEUE_MEMBER_COUNT(CurlyQueue)})
exten => 101,n,Noop(${QUEUE_MEMBER_COUNT(CurlyQueue)})
exten => 101,n,Noop(${QUEUE_MEMBER_COUNT(CurlyQueue)})
exten => 101,n,Hangup()

exten => 102,1,Answer()
exten => 102,n,Set(RING_CYCLE=${IF($[${ISNULL(${ARG3})} & ${EXISTS(${RING_CYCLE})}]?${RING_CYCLE})})
exten => 102,n,Hangup()

exten => 103,1,RaiseException(INVALID)
exten => 103,n,Playback(tt-monkeys)

exten => 104,1,Dial(Agent/1000)

exten => 105,1,Answer()
exten => 105,n,Set(Trapdoor=6)
exten => 105,n,While($[${Trapdoor}>0])
exten => 105,n,Playback(tt-weasels)
exten => 105,n,Set(Trapdoor=$[${Trapdoor}-1])
exten => 105,n,Noop(${Trapdoor})
exten => 105,n,EndWhile()
exten => 105,n,Hangup()

exten => e,1,Playback(tt-weasels)

exten => 123,1,Answer()
exten => 123,n,MixMonitor(/home/putnopvut/douglas.wav)
exten => 123,n,SayDigits(12345678901234567890)
exten => 123,n,StopMixMonitor()
exten => 123,n,Hangup()

exten => 124,1,Answer()
exten => 124,n,SayDigits(666)
exten => 124,n,Hangup()

exten => 1000,1,Dial(SIP/1000, 10)
exten => 1000,2,VoiceMail(1000@imapvoicemail,u)
exten => 1000,3,Hangup()

exten => 2000,1,Dial(SIP/2000,10,tT)
;exten => 2000,n,VoiceMail(2000@imapvoicemail,u)
;exten => 2000,n,Hangup()

exten => 3000,1,Dial(SIP/666&SIP/2000&SIP/1000, 10)
exten => 3000,n,Playback(tt-weasels)
exten => 3000,n,Hangup()

exten => 667,1,Answer()
exten => 667,2,VoiceMailMain(CALLERID(num))
exten => 667,3,Hangup()

exten => 5000,1,Queue(CurlyQueue,tT,,,10000)
exten => 5000,n,Noop(${QUEUESTATUS})
exten => 5000,n,Playback(tt-weasels)

exten => 5001,1,Answer()
exten => 5001,2,Queue(BitchQueue,t,,,10000)
exten => 5001,3,Noop(${QUEUESTATUS})

exten => 5002,1,Answer()
exten => 5002,2,SayNumber(${QUEUE_MEMBER_COUNT(CurlyQueue)})
exten => 5002,3,Hangup()

exten => 5003,1,Queue(HappyQ,t,,,10000)
exten => 5003,2,Noop(${QUEUESTATUS})

exten => 1001,1,Noop(${CUT(CHANNEL,,1)})
exten => 1001,2,AddQueueMember(CurlyQueue,${CUT(CHANNEL,,1)})

exten => 1002,1,RemoveQueueMember(CurlyQueue,${CUT(CHANNEL,,1)})

exten => 1003,1,AddQueueMember(BitchQueue,${CUT(CHANNEL,,1)})
exten => 1004,1,RemoveQueueMember(BitchQueue,${CUT(CHANNEL,,1)})

exten => 1005,1,AgentLogin(1000);

exten => 1006,1,AgentCallbackLogin(1000,,666@internal)
exten => 1006,n,Hangup

exten => 1009,1,AgentCallbackLogin(2000,,2000@internal)
exten => 1009,n,Hangup

exten => 1007,1,AddQueueMember(HappyQ,${CUT(CHANNEL,,1)})
exten => 1007,2,Hangup()

exten => 1008,1,RemoveQueueMember(HappyQ,${CUT(CHANNEL,,1)})

exten => 2001,1,PauseQueueMember(BitchQueue,${CUT(CHANNEL,,1)})
exten => 2001,2,Hangup()

exten => 2002,1,UnpauseQueueMember(BitchQueue,${CUT(CHANNEL,,1)})
exten => 2002,2,Hangup()

exten => _*10XXXX,1,Chanspy(SIP/${EXTEN:3}|b)
exten => _*10XXXX,2,Hangup()

exten => 42,1,Answer()
exten => 42,2,Playback(lyrics-louie-louie)

exten => 43,1,Answer()
exten => 43,n,Playback(dir-intro-fn)

exten => 6000,1,Answer()
exten => 6000,n,FollowMe(Mark,n)
exten => 6000,n,Hangup()

exten => h,1,Noop(${QUEUESTATUS})


I pick up a SIP phone and dial 666. Call is rejected with this message:
[Nov 12 15:06:06] NOTICE[19493]: chan_sip.c:14947 handle_request_invite: Call from '2000' to extension '666' rejected because extension not found.