Over two years after its release GTA V still has a few secrets. Those folks intent on solving the Mount Chiliad mystery don't seem all that insane anymore, do they?
A significant portion of GTA 5's easter eggs and cheats can be accessed via the in-game cellphone. Calling certain number combinations will result in some rather odd occurrences. Sometimes you'll get the iconic chirping of a dial-up modem, or some pissed guy telling you to 'stop prank calling' them (check out the full list).

Cheats can also be activated by calling certain numbers instead of the using regular button combo method. All of your regular cheats, like invincibility, drunk mode, slow-mo, fast run, and so on are accessible from the cellphone.
Recently, a new phone "effect" has been discovered. It isn't a cheat per-se (of course, it may have as of yet undiscovered effects), as it does not benefit the player in any way. If you call 1-999-367-3767 (1-999-EMP-DROP) the phone will show that you're connecting to someone or something called "Black Cellphones". All this does is trigger a small explosion in the air near you, but far enough not to hurt, and changes the color of your phone to black.