simply - depends on your definition of simplicity
you can have a computer acting as BT "end" for the phone and controlling different stuff
or you can use some micro controller with some BT 2 seral adapter (SENA makes them, for e.g. ESD110) ... you then program your uc to scan for available devices and when it recognises your BT device (phone) by it's MAC address - your uC do whatever you want it to do (beep, speak, turn lights on/off, control relay, shut down the alarm ..)
In general, a simple 1$ PIC (any one with serial port) will do the job + some BT module and some programming (not much)... problem is, BT modules will set you back some 50-100$, not sure you planned for that when you wrote "simple"