Yes buy a
People's sim in HK.
Last time I was there in HK I bought a prepaid People's sim and use it to call numbers in HK. Then I use my
Onesuite prepaid calling card for calling U.S. and other international numbers. Just dial Onesuite access number 1502-131 then your PIN and then the destination number (with the prefix 1 for US and Canada numbers and 011 for other countries.) It works in China too.