Deny call without answering

How can I deny incoming call without answering it?
I tried with Hangup(), but it doesn’t work!

exten => 1234/5678,1,Hangup()

You cannot hang up a call you haven’t answered. If all you phones are connected to Asterisk, simply not Dialing any extension should suffice. If you really want to hang up, try

exten => s,1,Answer() exten => s,n,Wait(1) exten => s,n,Hangup()

It is nicer to do:
exten => s,1,Answer()
exten => s,n,Wait(1)
exten => s,n,Playback(sorry_this_line_not_in_opertation) ; is name of soundfile
exten => s,n,Hangup()

If you don’t want to have incoming calls at all simply don’t add a start extension into our dialplan inbound context. (supposing that it is an external number from your telco or voip provider) That way callers will hear the “not in operation beep” from your telco or voip provider