The watchdog's investigations indicate that Elisa charges other operators "unreasonably high" local loop charges, which are not cost-oriented as stipulated by law. It has ordered Elisa to cut monthly loop fees by more than 20 percent.
The probe was carried out at the request of rival TeliaSonera.
Elisa criticised the decision and the calculation methods used, saying it will appeal to the Supreme Administrative Court.
The watchdog also said it is investigating the wider broadband market, aiming to bring prices down. It is probing five telecom operators on suspicion of irregularities in high-speed internet wholesale pricing.
FICORA added that it "has discovered flaws and problems in the pricing of several telecom operators" and suspects "unlawful activity" in the broadband market.
The regulator began its probe into the fees of five operators in February to lower broadband wholesale prices to "reasonable" levels and to prevent major players from monopolising prices.
Broadband Speeds Fall Short of Promises
In late April FICORA accused the nation's biggest telecoms providers of offering customers unreasonable -- and possibly illegal -- terms in fixed broadband contracts.
The agency says that the biggest problem with these contracts is related to advertised speed versus actual speed of internet connections.
It adds that the contracts are worded so vaguely that it is virtually impossible for a consumer to prove that the provided service does not match the speed promised in advance.
FICORA studied contract terms offered by DNA, Elisa, TeliaSonera and Welho.