Let me explain a bit more

Let's give an example
say, u have a HK number 60002345
the card number is 96954321 (first three number is 969, there's why the name of the card is "969")
ur number in china is 55544433322

when u're in china, but your fd in hk have ur hk mobile number only, how to solve this problem ?

The solution is
for your HK mobile (60002345), forward all ur call to the number 96954321 (96954321 is a HK number, no IDD is needed)
for the card "96954321", forward all ur call to the number 55544433322 (in this forwarding, only $0.3 is charged for each minute, no IDD charge)
Then, in china, using the number 55544433322, u can receive calls which ur fd calling at 60002345

feel free to ask if you can't catch how to use this