How to ignore a call

I have an SPA3000 on my house line, with my house phones on the FXS port. Any incoming call from the telco rings thru to the house phones right away. Fine - I want that. There is an answering machine in my phone that I want calls to go to most of the time. I have the SPA3000 set to make a PSTN-to-SIP call after a 30 second delay to make sure that answering machine gets its chance.

But I want Asterisk to sometimes handle a call right away. For example, if I want the call forwarded somewhere else according to my follow-me mechanism.

If I reduce that PSTN/SIP delay to 2 seconds (give the SPA3000 time to decode the CID) it calls Asterisk while the house phone is still ringing. And then if Asterisk has no forwarding instructions how do I make it ignore the call so it does not fall off the end and trigger the “number is not available” recording? In such a case I would rather the SPA3000 got no response to its INVITE at all and let the house phone deal with it.