You need to pay a deposit for gas, electricity and water. They are in the region of $600 each from what I recall. These are payable in your first bill so 1-2 months after arrival.
You will need 3.5 times your rent in the first instance: 1 month deposit, 2 months rent in advance and 0.5 months for the agency fee.
You can trade in your current phone to get a discount on a new one. You could also just get a new SIM (PAYG) for about $60 for the time being. I just got one for a friend who was visiting and calls back home were charged at the same rate as local. Your phone will need to be unlocked though. It's only UK phones that are locked as far as I know but you can get that done in various places. For a new phone with contract, depends what your budget is like but you can get them for under $500. You usually need to pay a deposit for IDD (international calls), I paid $3000, and perhaps your first months rental (under $100).
HKID is free...just a couple of boring hours at immigration tower.
Hope this helps!