Need ideas for this: frequency follower robot

i am thinking of making a robot that responds when a certain device creates a signal, like when you are requesting for the robot to come. the robot will then automatically go to the source of the signal. it will have a proximity sensor for barricade avoidance. how will it be possible and what should i use? thanks! im just a newbie in the robotics world.

if you need just a signal or frequency following rob, you can use plls.. or if you want more advance ones, you can use voice recognition chips which a a bit costly but worth every penny..