79 switched network.’” Vonage Holdings Corp. Comments at 43, GN Dkt. Nos. 14-28 & 10-127 (July 18, 2014). Mobile petitioner CTIA responded to that point in its reply comments, disputing Vonage’s underlying assumption that mobile broadband users can connect with all telephone users, see CTIA Reply Comments at 45, GN Dkt. Nos. 14-28 & 10-127 (Sept. 15, 2014), thereby recognizing that the definition of public switched network was in play. In addition, over the course of several months before finalization and release of the Order, mobile petitioners (and others) submitted multiple letters to the Commission concerning the potential for redefining the public switched network. See, e.g., Letter from Henry G. Hultquist, AT&T, to Marlene H. Dortch, FCC, GN Dkt. Nos. 14-28 & 10-127 (Feb. 13, 2015) (“AT&T 2/13 Letter”); Letter from Scott Bergmann, CTIA, to Marlene H. Dortch, FCC, at 13-18, GN Dkt. Nos. 14-28 & 10-127 (Feb. 10, 2015); Letter from Scott Bergmann, CTIA, to Marlene H. Dortch, FCC, GN Dkt. Nos. 14-28 & 10-127 (Jan. 14, 2015) (“CTIA 1/14 Letter”); Letter from Gary L. Phillips, AT&T, to Marlene H. Dortch, FCC, GN Dkt. Nos. 14-28 & 10-127 (Feb. 2, 2015); Letter from Scott Bergmann, CTIA, to Marlene H. Dortch, FCC, GN Dkt. Nos. 14-28 & 10-127 (Dec. 22, 2014) (“CTIA 12/22 Letter”); Letter from Scott Bergmann, CTIA, to Marlene H. Dortch, FCC, GN Dkt. Nos. 14-28 & 10-127 (Oct. 17, 2014) (“CTIA 10/17 Letter”). We have previously charged petitioners challenging an agency rule with actual notice based on letters like those submitted by mobile petitioners. See Sierra Club v. Costle, 657 F.2d 298, 355 (D.C. Cir. 1981). But we have even more evidence of actual notice here. Mobile petitioners note in their letters that, in meetings with the Commission, they discussed the substance of their arguments here, including