The case against Vodacom is backed by funders seeking a return on their investment.
Vodacom on Tuesday put into effect new call termination rates following a ruling by the Johannesburg high court.
The court on Monday found the proposed Independent Communications Authority of South Africa (Icasa)​ rates to be invalid and unlawful, but has given Icasa six months in which to amend its regulations, during which time the proposed rates stand.
Vodacom spokesperson Richard Boorman confirmed that the rates had been implemented and said the company was studying the ruling and would comment further in due course.
"Our legal challenge has been justified, the view that the call termination regulations are unlawful has been vindicated," he said.
MTN and Vodacom took Icasa to court to stop it from implementing a regulation on cellphone termination rates. These are the rates operators pay one another for calls to other networks.
"MTN awaits a copy of the written judgment, whereafter it will consider its options going forward," MTN chief executive Zunaid Bulbulia said on Tuesday.
Icasa said MTN was required to comply with the ruling.
"The ruling is out, all of us must comply," Icasa spokesperson Paseka Maleka said.
He said the operators were expected to apply the new call termination rates for six months, while Icasa amended its regulations.
"If they do not comply, we will take them to the complaint and compliance committee to compel them."
MTN and Vodacom have to pay 44c a minute to smaller operators, while the smaller companies have to pay only 20c. – Sapa