Answering machine program?

[Matrix]

Hello-

I've been looking for a program that would allow me to record a message for a certain caller, and automatically answer/play it when they called. Kind of like client-side voicemail.

I googled, etc, but I can't find anything. Is such a thing even possible? I think I remember reading somewhere that WM devices can't play sounds on an active phone call.

Thanks for any suggestions!

-TM

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Update:
I guess being up 30 hours has its downsides...it just occurred to me, what I'm looking for is simply a software answering machine.