Colo. Rev. Stat. § 40-4-104
Whenever the commission, after a hearing upon its own motion or upon complaint, finds that a physical connection can reasonably be made between the lines of two or more noncompetitive telephone public utilities whose lines can be made to form a continuous line of communication by the construction and maintenance of suitable connections for the transmission of messages or conversations and that the public convenience and necessity will be served or finds that two or more telephone public utilities have failed to establish joint rates, tolls, or charges for service by or over said lines and that joint rates, tolls, or charges ought to be established, the commission may by its order require that such connections be made and that conversations be transmitted and messages transferred over such connection under such rules as the commission may establish and prescribe through lines and joint rates, tolls, and charges to be made and used, observed, and in force in the future. If such telephone public utilities do not agree upon the division between them of the joint cost of the physical connection or connections or the division of the joint rates, tolls, or charges established by the commission over such through lines, the commission has authority, after further hearing, to establish the division by supplemental order.
C.R.S. § 40-4-104
For duty of telephone company to transmit messages of another company, see § 40-3-107.