> 1- a user in China call a number in the US. after the first ring the user hang up
There is a high chance that his ANI will not be detected by the server in USA. E.g. US gateway will receive something like 04532324, which is a local number in the area customer is China – but there is no way to tell which province it is in.
> is it possible to restricate the callingback to a specific phone numbers
Of course, it is all up to the callback server – how you will set it up.
> is there an “antiflood” system to prevent some ppl from keep calling and calling the callback number?
See above.