I know this one is probably simple, but after a bit of searching “the Googs”, I wasn’t able to find what I thought was the answer, so here goes…
How can I have the system NOT play the Number Not In Service message when an unknown DID is delivered? Instead, I’d like the system to send a 404 back, so an upstream system can try other trunk options. However, because the message plays, the upstream system considers the call “answered”.
Thoughts?
Just thinking while I type:
Add a new context and point your “any/any” route at it.
Set the new context to NOT answer the line, but have it send the 404 back.
That last bit is the only piece I’m not sure how to do - there are probably context examples out there that can tell you how to send a 404 back instead of answering the call.
OK, I can honestly say, I have that module installed but it has always scared me a bit. 
So, I created a context called “noaccess”, with everything set to deny (hoping that would even deny access to internal announcements). I then pointed it to that context, and now it’s telling me my call can’t be completed as dialed. It looks like it’s getting a 487 instead. How would I be able to force a different response?
post it and let’s see what we can do.
I don’t know how you could do with with dial plan. In order to execute any dial plan, the SIP Invite-OK-ACK has already occurred, at that point it’s too late to present a 404.