What is the little hole besides the headphone jack on the iphone 4?

Okay i have heard people with att that has an iphone say that if they stick a pin in the hole that a dim card tray will pop out. But my iphone is with sprint so i thought they didn't have sim cards so what will happen if i hit the button.

* i'm not trolling this is my first time having anything from apple i'm a android person.
* please no smart remarks!

If the hole has an oval around it than its where a sim card is stored.other holes might be speakers and mics so don't go pokin them all.
sim card doesn't matter the company, just the model of phone

tiny hole by headset hole on iphone4 is a mic

It is a secondary microphone. Usually used for noise cancellation, or for video sound recording.

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