BT is owed millions of pounds from UK mobile operators who tried to block increased costs for customers dialling 0800 and 0845 numbers.
The Supreme Court said mobile operators must pay the shortfall, ruling against appeals filed by the parent companies of UK operators including Vodafone, EE, Three and O2.
EE said: "We are both surprised and disappointed by this decision. We are closely considering our position, but, regardless of the outcome, EE remains firmly of the view that BT's ladder charges are ultimately harmful to consumers and investment in mobile coverage."
The Supreme Court said BT could claim retrospective costs from 2009 - when it introduced 'ladder costs' for calls to 08 prefix numbers - totalling "tens of millions of pounds", according to a telecoms executive who spoke to The Financial Times.
Following the decision to increase call charges five years ago, mobile operators challenged BT via Ofcom, which agreed the ladder charges were unreasonable and mobile operators were refunded for their customers' calls to the premum numbers.
BT responded by referring the case to the Competition Appeal Tribunal (CAT), which passed the disagreement to the Supreme Court.
The Supreme Court said Ofcom had taken the incorrect action by saying the rising costs should be passed onto customers.
A BT statement said: "We will now start proceedings to recover the money that has been refunded to the mobile operators since the Court of Appeal ruling. We will also be pursuing claims for further termination charges subsequent to that ruling...Such pricing was designed to benefit UK consumers by incentivising the mobile operators to lower their retail prices."
