Thank you, the green tables do look helpful, although it's the same frequencies from the jeppessen, just listed in an easier to find format as far as I can tell. HF option is nicely laid out in the green tables on the ERC2 chart too.
The 3 minute call aheads to Hong Kong have been straightforward because they always answer, no problem there, same with Vientiane. With sanya and guangzhou I am not usually successful in getting a response when I first start calling 20 minutes out, but usually it works within 5-10 minutes of the FIR boundary. I start with the appropriate sector frequency then start calling other nearby sectors if no answer. It might be a VHF reception issue.
In regard to the reason, I was more curious about the reason for needing to call 5 minutes ahead to Sanya when coming from Guangzhou southwestbound, as I thought that within the PRC such a thing wouldn't be necessary, although it's just my curiosity. As long as the requirement is listed I will continue to comply as best I can.