All local fixed network operators offering telephone services are required to provide unified telephone directory service. You could enquire all the fixed-line telephone numbers operated by different operators through unified hotline numbers: 1081 (English), 1083 (Cantonese) and 1088 (Putonghua).
As far as we know, customers are registered in the unified telephone directory service automatically unless they have instructed their service operators to have their numbers removed from the said service.