We drove up from Phoenix to Flagstaff and went to the south rim. We had a party of people meeting and ironically the fed govt was in shutdown mode if you remember. We were doubtful we would be able to get in. Miraculously the day we were driving up to flagstaff it was announced the shutdown was over with and we were able to get into the park.As I recall from my visit last summer, for the north rim of the Grand Canyon, nothing and I didn't miss it. Coverage pretty much stops shortly south of Jacob Lake where you turn off of 89A on to 67 into the park. There is a Verizon tower close to the intersection which also provides voice only service to ATT. As for other comments saying what about if your car breaks down.... There is only one road and if your car breaks down, someone else will come along. We survived before cell phones and get along without them quite nicely. Go outside and enjoy the ultra HD view.
The biggest issue is getting a campground spot. Reservations are required in the park. Winging it is no longer allowed. If you don't have reservations you camp at the commercial or USFS campgrounds at Jacob Lake or boondock between Jacob Lake and the park.
The store at the campground has limited WiFi if you are in the store or seated on the porch with your phone held the right way. I used it to send an update to my family as I was traveling solo. The pay phones have been removed.
We took the blue?, red?, yellow? line tour of the south rim and VZW service was spotty to non-existent after the start of the tour. There seemed to be service at the rest areas with food.
We were at one lookout about a half hour into the tour. This outlook point had a rock to climb up and get a better view. No sooner did I climb to the top for the view when my phone rang. It was a business call which I took for a half hour standing on top of this (huge) rock in one spot, voice quality was perfect. After the call I climbed down from the rock and there was virtually no service again. While I wish I could disconnect totally, I was glad to be able to get that one call and billed the client for my time.
Yes we survived without cell phones, and I can remember rotary phones, but it's nice to know one can get a connection even in the middle of nowhere.