Don't listen to the rubbish above, call ahead. China has introduced demerit points and fines. Most ATC units will hand you off early, but the responsibility is the PIC.
The reason for the call ahead is that the ATC units don't actually talk to each other, there is no flight strip passed from one FIR to the next, e.g. from Vietnam to Sanya. Have two radios turned up, one called ahead.
The requirements for contacting HKG are listed in the AIP
http://www.hkatc.gov.hk/HK_AIP/AIP/ENR/HK_ENR1.1.pdf
As for knowing what frequency to contact a ATC on each airway in China at the FIR, China has its eAIP and charts online as PDF files. The chart you want to look at is ERC 2 for Sanya which has the frequencies listed as a table in green for each route.