HK mobile market quite competitive so you have too much choice in sim cards
How long you staying for ? Only HK or china too ( not sure if worth while get 1 sim 2/3 numbers package.... Then data for those 2/3 too.... )
Else if just HK , yeah russex's info is good, HK is very well served by wifi , most shopping malls , Mac D , all MTR station, , strangely I'm finding the iPhone still register GPS even without 3G signal , so if you preloaded your map area into cache , still usable
Walkup are pink light , in old builings, sometime mixed into residential buildings, 1 WG per door, lots of ring door bells. I'd say about 60-70% accurate of girl from DB is there and not some old ads / BS
Hotel "walk" or HG are hotel girls usually very hot , model types , and yes , you call , make appointment , go to hotel , reaffirm your arrival , then mms will tell you which floor / rm number , you knock , hopefully not BS, if you happy they happy > you very happy !
There are other 'call up' 400-600 , and done in residential buildings , not sure how these are classified. But similar drill as HG , just not as model like , service still depends on your charisma , regardless WG,HG,... Streetwalker