TELEPHONE numbers will be unified to eight digits on January 1, 1995, as part of the territory's numbering plan outlined for the coming 15 years. The draft plan, revealed yesterday by the Office of the Telecommunications Authority (OFTA), would also put Hong Kong among the world's first areas to attempt number portability. ''We welcome comments from the industry on the draft plan from now, and expect to implement the numbering change on January 1, 1995,'' said OFTA director-general Alexander Arena. The switch to eight digits would mean adding a figure to the front of all residential and business numbers. Mobile phone numbers would remain unchanged and radio paging numbers would be cut by a digit. Telephone numbers would begin with the digit 2 - overthrowing to digit 3 when necessary - radio paging numbers with 7, personal numbering 8, and mobile phones 9. The prefix digit 8 - considered the golden or lucky number by the Chinese - is given to the future personal numbering system, as it can access all forms of fixed or mobile services.