You called ATC, and then you waited. And waited. And you didn't get an answer back. It's not that ATC wasn't paying attention. They were busy doing other things, like this...

1) Your controller is running a second frequency

Ground controllers often run the clearance delivery frequency as well. If someone calls up before you on the other frequency, they'll get priory. And unless you're monitoring the other frequency, you won't hear anything until ground gets back to you (or you call again when they're done).

2) Your controller is entering a flight following request

If you call up clearance delivery and requestion flight following, the controller needs to key your flight information into their computer, including your tail number, destination, and altitude.

And to make matters much worse, they need to enter the information on a non-QWERTY keyboard.

If they're busy entering flight information, they might not respond to other aircraft as they're hunting-and-pecking their way through the flight following request.

3) Your controller was on the "land line"

So what exactly is the land line, and what is your controller doing on it?

Controllers have a phone to talk to other controllers. They'll make calls for a lot of reasons, like coordinating aircraft transitioning into each other's airspace.

And for IFR departures, tower controllers will often call approach controllers to get departure headings for the aircraft getting ready for take off.

So the next time you call ATC and don't get an answer, give them a minute before you call back. They're probably busy on another frequency or phone.