I've just made a mobile phone hacking project last week. It works like this; an mcu is connected to a mobile phone, and the phone can be placed anywhere you want. then, when you 'miss call' the number, the phone will detect your call, take a picture with its camera, and then send the picture back to you.
the wiring to the phone was pretty simple, though. we just soldered some wires on to the phone's keypad tracks. that way, we can control the functions of the phone by connecting two (or more) wires together. and an LDR is placed on the keypad LED to detect when the phone receives a call.
so yes, the process is pretty much irreversible. ie you cant use the phone as a normal phone anymore if you do it this way. anyway, it was not the intention of the project to make a robust survaillence device. we just wanted to demonstrate the use of MCUs and probably get some high school kids to do the same.
feel free to ask anything