Call/Text Alert Project

Hello,

I recently learned about low and high pass filters and I believe there's no better way of fully understanding the topic until you put it in practice. I'm pretty sure someone's already made this, but I cannot seem to find it, but how do I start working on an Arduino project where there's a circuit connected to a breadboard and an LED flashes once a sensor picks up a frequency from an incoming call or text from your phone. Any guidance would be awesome and very much appreciated :)

Google for "cell phone detection using arduino" to get some ideas but don't be disappointed if it does not give you an opportunity to practice all your filter design knowledge.