Arbitration clause in a fixed and mobile telephony services concession contract. Clause that pertains solely to Chapter XIII of the contract, "which deals with tariff revision", and does not encompass disputes relating to Chapter XII, concerning "mere tariff adjustment", which is the subject matter under judicial discussion. Furthermore, "the arbitration clause does not preclude access to the judiciary, as it merely makes the use of arbitration proceedings optional". Rejection of the objection based on an arbitration agreement. Continuation of the lawsuit: