The telecommunications regulator has ordered small fixed-line carrier Wharf T&T to detail its network access within two weeks or face a fine.
The Office of the Telecommunications Authority made the order after Wharf T&T refused to provide it with a list of buildings connected to its network.
Ofta said Wharf T&T had cited the need to protect commercial secrets as the justification for its refusal. It did not accept Wharf T&T's argument.
Three other small carriers - Hutchison Global Communications, New World Telecommunications and Hong Kong Broadband Network - submitted network details to Ofta.
The regulator needs to identify which buildings have at least two self-built networks, including that of dominant carrier PCCW, in order to start a gradual roll-back of the mandatory Type II interconnection policy.
Under a policy announced by Ofta in July, any buildings that had at least two self-built networks could gradually phase out the mandatory Type II policy in three years, meaning PCCW would no longer be obligated to lease its last-mile capacity to rivals.