Consulting on Switch system.

Hello...! guys
I have started my project deals with a system known as doorbell watcher, this system is intended to be used when a person leaves the house for a few hours or a week-end. The system conceiving to allow a person to monitor if some one comes to the home place and ring the doorbell the system will tell to leave the message through a microphone which will be found at that sytem.This system would record the message on a tape through a tape recorder,The operation of this system is related to the mobilephones system on voice mail when you are told to leave a message on some body phone's.
So my big problem on my system is how can I design a switch that can compare the two signals ? 1.Sense a signal from the doorbell and connect to the memory chip 2. To sense a signal from a microphone and that switch connecting to the tape recorder when a person start talking.

Thanks.
Gombo :?:

Mr gombo what did you say your system do ,is it a telephone message allert or a door alarm that will record any passerby?please elaborate more