Where on that page does it show that it is a necessity? I haven’t been to Rocky Mountain recently (and when I did go right before COVID don’t need one) but am an avid National Parks visitor and have never once needed a phone. You can print the timed entry codes (which I often do because of lack of cell service in some).

>"and have never once needed a phone. You can print the timed entry codes (which I often do because of lack of cell service in some)."

I mean OK, fair enough; it's not NEEDED to have a phone. But short of printing out, they heavily incentivize timed entry codes being shared by phone; either by screenshot or sharing your phone for the code.

When we do group visits to Estes Park from here in Denver, all members of the group use a phone.