Open up Niko's phone, and dial 911.

A voice will then start talking to you, and will say: "Hello, you're through to the Liberty City Emergency Services. Use your touchtone keypad to select which service you require. Press 1 for police, 2 for paramedics, or 3 for the fire department".

Once he has said all that, choose one (I go with police). once you have selected, you will hear sirens in the distance.

The service you chose will come, thinking there really is an emergency - but all this time it was you making a prank call!