220 ILCS 5/13-507
In any proceeding permitting, approving, investigating, or establishing rates, charges, classifications, or tariffs for telecommunications services offered or provided by a telecommunications carrier that offers or provides both noncompetitive and competitive services, the Commission shall not allow any subsidy of competitive services or nonregulated activities by noncompetitive services. In the event that facilities are utilized or expenses are incurred for the provision of both competitive and noncompetitive services, the Commission shall apportion the facilities and expenses between noncompetitive services in the aggregate and competitive services in the aggregate and shall allow or establish rates or charges for the noncompetitive services which reflect only that portion of the facilities or expenses that it finds to be properly and reasonably apportioned to noncompetitive services. An apportionment of facilities or expenses between competitive and noncompetitive services, together with any corresponding rate changes, shall be made in general rate proceedings and in other proceedings, including service classification proceedings, that are necessary to ensure against any subsidy of competitive services by noncompetitive services. The Commission shall have the power to take or require such action as is necessary to ensure that rates or charges for noncompetitive services reflect only the value of facilities, or portion thereof, used and useful, and the expenses or portion thereof reasonably and prudently incurred, for the provision of the noncompetitive services. The Commission may, in such event, also establish, by rule, any additional procedures, rules, regulations, or mechanisms necessary to identify and properly account for the value or amount of such facilities or expenses.
The Commission may establish, by rule, appropriate methods for ensuring against cross-subsidization between competitive services and noncompetitive services as required under this Article, including appropriate methods for calculating the long-run service incremental costs of providing any telecommunications service and, when appropriate, group of services and methods for apportioning between noncompetitive services in the aggregate and competitive services in the aggregate the value of facilities utilized and expenses incurred to provide both competitive and noncompetitive services, for example, common overheads that are not accounted for in the long-run service incremental costs of individual services or groups of services. The Commission may order any telecommunications carrier to conduct a long-run service incremental cost study and to provide the results thereof to the Commission. Any cost study provided to the Commission pursuant to the provisions of this Section may, in the Commission's discretion, be accorded proprietary treatment. In addition to the requirements of subsection (c) of Section 13-502 and of Section 13-505.1 applicable to the rates and charges for individual competitive services, the aggregate gross revenues of all competitive services shall be equal to or greater than the sum of the long-run service incremental costs for all competitive services as a group and the value of other facilities and expenses apportioned to competitive services as a group under this Section.
220 ILCS 5/13-507