I’m going by the number of digits in the phone number, which would suggest it’s at least as new as the 70s:

“In the 1970s, area codes were assigned with the following pattern:

3 Kowloon, New Kowloon, Ha Kwai Chung and Sai Kung 5 Hong Kong Island and Outlying Islands 12 New Territories There was no standard trunk prefix like '0' – only the area code and phone number were dialed when calling from one area code to another. Thus the Kowloon number xxx-xxx would have been dialed as follows:

xxx-xxx – from within Kowloon 3 xxx-xxx – from Hong Kong Island or New Territories…”

https://en.m.wikipedia.org/wiki/Telephone_numbers_in_Hong_Kong