The most direct way for a pilot to communicate with air traffic control (ATC) is by speaking to them over the radio. Sometimes, though, it's hard to get a word in, "edgewise," when ATC is busy speaking to many different aircraft in rapid succession. This can be an issue when a pilot is trying to establish initial contact.
It's not uncommon, under busy circumstances, to hear ATC tell a pilot, who is trying to establish initial contact, to call back later, perhaps in two or ten minutes. That's happened to me, a couple times, as I orbited the Del Mar Racetrack waiting for permission to enter the busy "bravo" airspace surrounding Miramar Airport and San Diego's Lindbergh Field.
Occasionally, there are times when I've established communications with ATC, but, due to their workload, I'm not receiving timely updates. The delays aren't a safety issue, but I often wonder if the controller has overlooked me or if I'm simply a lower priority.
The corporate pilot for QUALCOMM gave me a simple trick for getting the controller's attention. She told me to simply "ident" which causes my plane to flash on the controller's radar screen. Ident is short for "squawk identification." It's a signal sent from a plane's transponder to help a controller identify an aircraft's secondary radar (transponder) return. By asking a pilot to "squawk ident," the controller can ensure that the aircraft they're talking to matches the radar target they're looking at.
Airplane transponders send out codes that the pilot sets. The transponder code 1200 is the most common code when a pilot is flying VFR. If a pilot is flying under an instrument flight rules plan (IFR) then the controller will assign a specific code to an aircraft to track the callsign and aircraft type.
ATC has established specific codes for special situations:
7600 Communications failure
7777 Military intercept
Simply hitting the ident button is a helpful way to get ATC's attention. I like simple tricks like this because they're effective yet not as brittle as an unbaked computer hack.