Is this Confirm Calls behavior possible?

Is it possible to use “Confirm Calls” only for X seconds (or X rings)?

I have my DID set up to ringall my extension and cell phone. Works great, except when my cell phone is off (or has no signal), then the DID call goes straight to my cell voicemail.

Incomplete Workaround:
If I check Confirm Calls, then after not answering my cell phone or extension, the call goes to cell voicemail (what I want), but voicemail can’t be left by the caller (call rings until hang up and voicemail just plays the Confirm Calls message).

What I Want:
So is it possible to have Confirm Calls disable after ~4 seconds? That way if the no-cell-signal VM picks up, my extension continues to ring, and if I don’t pick up my cell phone, the caller goes to cell phone voicemail normally.

Is It Possible?:
I believe Google Voice does it this way too. If you have your Google Voice number forwarded to a cell phone and you answer your cell phone immediately when somebody calls your GV number, then you’re prompted to “Press 1 to answer”. You don’t get the prompt if you answer after a few seconds of ringing.