Beacuse if it does work it's cheaper than making a call to +852 2747 5500, which is always (as far as I know) published alongside the +800 equivalent, and which will work from anywhere.

Many companies (particularly hotel groups and so on) have such numbers - they are known officially as Universal International Freephone Numbers (UIFN). Unfortunately, the extent to which they are actually implemented by telcos varies enormously around the world. As a general rule of thumb, fixed lines work better than mobiles (and if it works from a fixed line then it will definitely be free to the caller). If on a mobile then sometimes only one IDD provider will provide access (for example, in Thailand 001-800-2747-5500 tends to work, but 007-800-2747-5000 doesn't).