I have been using a 3 sim card for about 2 years. One of my suppliers got it for me, along with a China Mobile sim card and a cheap Nokia phone.
The calls are so cheap with 3 from HK it is stupid. The money expires after 6 months, and I generally come to HK twice per year, so I have to re-charge each trip (I do not want to lose the phone number). At 7-11 and other convenience stores, I can only find HK$100 re-charge cards.
I cannot use HK$100 in a trip. I can call home (USA) every day and the office almost every day, plus other calls, and still do not even come close to using it up.
Does anyone know where I can get a card smaller than HK$100? I have accumulated about HK$400 - it seems crazy to keep adding every trip.