In order to call the police, you will need to locate the telephone, which will be in one of the cabins around the map (which cabin has the phone will change from game to game). Approach the phone and hold the A button to call the police.
Usually, but not always, the phone box will be destroyed at the beginning of a match, and will need repair before you can dial. Jason can also destroy the phone box if it is in working order.
The box is connected to outside of the cabin with the telephone in it. In order to repair it, you'll need to find a fuse. Just like all repair parts, it is hidden randomly in one of the cabins, however, it is a small item and will be located inside of a drawer or cabinet. It can also be carried alongside a weapon, unlike other repair parts.
Once the phone box has been repaired and the police have been called, a countdown timer will appear in the upper left hand corner, visible to all players (including Jason). It will take five minutes for the police to arrive. When the countdown timer reaches zero, the police will appear just outside of the map in one of the two camp exits - the same ones you would use to escape in a car. You can see the police car on your map once they arrive, making it simple to find your way to the correct exit, but beware as Jason can also see the police car on the map, and the police will not fire at him or impede him from attacking you in any way.