The Commerce Commission will seek leave to appeal to the Supreme Court to try to overturn Court of Appeal and High Court rulings that cleared Telecom of breaching the Commerce Act when it introduced an 0867 number for dial-up internet calls in 1999.
The commission's general counsel, Peter Taylor, said the earlier rulings relied on case law developed by the Privy Council in 2004 that posed "real difficulties" for the commission and was affecting its ability to promote competition.
Telecom spokesman Mark Watts says the firm is disappointed the matter is going so far "at great expense".
However, Telecom remains confident in its arguments, Watts says.
The commission spent $521,592 unsuccessfully appealing against the High Court ruling.
Telecom introduced the 0867 prefix for dial-up internet calls to remove what the High Court last year described as a "perverse incentive" for rival internet providers (ISPs) to make dial-up internet cheap or free.