63 mobile service, is still subject to common carrier treatment as the “functional equivalent” of a commercial mobile service. See 47 U.S.C. § 332(d)(3); 2015 Open Internet Order, 30 FCC Rcd. at 5788–90 ¶¶ 404–08. 1. We first consider mobile petitioners’ challenge to the Commission’s updated definition of “public switched network.” That term, as set out above, forms an integral component of the statutory definition of “commercial mobile service.” Any such service must qualify as an “interconnected service,” defined in the statute as “service that is interconnected with the public switched network.” 47 U.S.C. § 332(d)(1)–(2). And Congress expressly gave the Commission the authority to define the public switched network, id. § 332(d)(2), which the Commission exercised by revising its definition in the Order. As we have explained, the Commission, relying on the growing universality of mobile broadband as a medium of communication for the public, expanded the definition of the public switched network so that it now uses IP addresses in addition to telephone numbers in connection with the provision of switched services. Mobile petitioners argue that Congress intended “public switched network” to mean—forever—“public switched telephone network,” and that the Commission thus lacks authority to expand the definition of the network to include endpoints other than telephone numbers. We are unpersuaded. Mobile petitioners’ interpretation necessarily contemplates adding a critical word (“telephone”) that Congress left out of the statute, an unpromising avenue for an argument about the meaning of the words Congress used. See, e.g., Adirondack Medical Center v. Sebelius, 740 F.3d 692, 699–700 (D.C. Cir. 2014); Public Citizen, Inc. v. Rubber Manufacturers Ass’n, 533 F.3d 810, 816 (D.C. Cir. 2008). If