Like in most of Asia, cell phones are widely spread and widely used with very competitive prices between providers of services.
Here is a list of cell phone providers:
Like most of the world, Hong Kong uses the GSM cellular phone system and you have to be aware that for some options you pay the incoming calls like you do in North America.
On De Voeux Road between N° 121 and 135 you will find the five main cell phone companies in Hong Kong.
It might also be worth getting a prepaid card that comes with a phone. A typical cost for that would be between 200 and 300 HK$ with some SMS and calling time included. Of course, being Hong Kong, you would get all the latest cell phones on the market!