Prefix when dialing International Numbers?

Channelman

When I use the 0845 type numbers of Cleverates, DiscountDial or
Best Minutes I understand that I need to dial 0044 xxx xxx xxxx # since
they all give this as a example, but what if I want to dial the USA?
How many zeros do I use? 001 or 0001. Also Hong Kong with it's
prefix code of 852. I can't use 0852 but is 00852 correct?
AdvTHANKSance, Channelman

Steve_xx

To dial

USA 001
Hong Kong 00 852


Use 00 to prefix all international codes.