The crew will press some of the buttons on the radio (same buttons as you'd find on a telephone) to key up the dispatcher. On CP, the "beep beep" is acknowledging that the closest radio base (what the dispatcher uses to talk to the trains) heard the train key it up, and the three "rings" confirms that the dispatcher got the message. Occasionally, if the dispatcher doesn't get the message for some reason, you'll hear some erratic sounds (sort of like a busy signal on a phone). This is just CP, mind you, though CN's procedures to key up the dispatcher are just as complex. On most railroads, you just hit a few buttons, and it'll beep one time, acknowledging that the base heard it and it was passed along to the dispatcher - no potentially "rejecting" it like the CP (as I explained above). To put it all simply, anywhere you are, these beeps and tones simply means someone, somewhere wants to talk to the dispatcher.