Dialplan:

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

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.