Taiwan’s telecoms regulator has fined Taiwan Mobile and Asia Pacific Telecom (APT) each TWD300,000 (about $9,500) for not informing it about their 4G roaming partnership within the required timeframe.

The two firms signed a roaming deal in September, but the agreement didn’t specify the costs, which were not worked out until December, the Taipei Times reported. The National Communications Commission (NCC) said the agreement, even without pricing details, should have been reported within a month.

In addition to the roaming investigation, which involved APT sharing Taiwan Mobile’s 4G network where it doesn’t yet have coverage, the NCC is looking into whether the lower rates charged by APT violate fair trade laws.

The NCC said other possible violations include changing the 4G technology used (it dropped plans to offer VoLTE) and not deploying the 2,000 base stations indicated in its business plan, so additional fines are likely, the Times said. Those penalties could range from NTD300,000 to NTD3 million.

In late January NCC said it was investigating complaints by three rivals that the 4G roaming agreement between APT and Taiwan Mobile was illegal. Chunghwa Telecom, Far EasTone Telecommunications and Taiwan Star Telecom alleged that APT, which has a 4G licence, was using Taiwan Mobile’s network and has not built its own 4G network.