Mobile Phones in Hong Kong
Unusually for such an expensive city, mobile communications in Hong Kong are very cheap. Coverage is just about universal on the bigger networks and many people opt to use a contract SIM rather than a land line at home. Coverage varies between networks, as do service features but in general, coverage is very good all over Hong Kong. There is even a signal in underground stations and cross harbour tunnels. There are also a vast array of prepaid options and promotions available to everyone.
Personally, I use SmarTone, who are one of the big networks with outstanding coverage. The signal is usually so strong that it even penetrated elevators and worked in basement bars and clubs.
There are however other networks and you should study the latest deals and promotions.
Prepaid SIM cards can be purchased everywhere and all branches of 7-Eleven for example have cards from all networks. This is a good option if you are bringing your existing phone with you and need to get set up with a local number quickly.
Competition is so fierce, there is usually little to distinguish between the offers and many people simply go for the best network coverage... and as for expats, the international call rates are often the most important consideration. Most SIM cards will cost around HK$100 and come with some stored credit.
Long term, it will likely be more cost effective for you to go to a contract SIM card, or annual plan. If you already have a prepaid SIM with a number you like, you won't lose this number if you go to a contract plan, as number portability is available, even between operators.
It's worth noting that in Hong Kong, it is often the person receiving the call on their mobile that pays for the call but it is so cheap it's hardly an issue.
Will my phone work in Hong Kong?
The answer is almost certainly yes. However, phones purchased in some countries on certain networks (UK being one example) may be locked to their home network. If that is the case, it is quite straightforward to get that phone unlocked to work on any network in Hong Kong.