Up in the Sawtooth mountains, in and around the Stanley area. Verizon does not work, but I have heard ATT does; I'll find out in a few months since I just switched. It appears ATT might actually have 3G in that area. However, Verizon phone service was available via Onstar.
Between Cody WY and East entrance to Yellowstone; one actual campground is Wapiti a FS campground (with power).
Off the beartooth parkway in most locations, even those campground before the climb up to the switchbacks, just west of Red Lodge. I was able to use Onstar phone a few times in some locations.
Anywhere I have camped in the San Rafael Swell in UT. NOTHING WORKS....Even XM can be blocked by the cliffs.
Gila Gulch or anywhere off the Black Hills Byway, south of Clifton AZ
Green River overlook, about 10 miles west of Dubinky Wells Road; northwest of Moab
And some that I used to be able to access, but now without a Hummer....might not be possible, or I might not want to try....
Corkscrew Gulch south of Ouray CO. Great place to plug a trailer tire and blow it back onto the rim with WD-40.
(and for the tire police, it was mounted as an emergency spare, which thankfully I didn't need)
Cathedral Valley in Capital Reef NP,
Colorado Overlook, off Anticline Road, Canyon Rims, south of Moab (destroyed two tires; had to leave the trailer sit overnight till I returned with a few new tires....reason I ditched the STs and went with truck tires....)
Sand Island, south of Bluff UT. Heard ATT works, but could not get anything on my Motorola Razr Maxx
Valley of the Gods, south of Bluff UT, just north of Mexican Hat...some areas do get phone service; especially when you get closer to the Goosenecks State Park or when you get up on top of Cedar Mesa via the Moki Dugway (a great switchback from the valley of the gods up to Cedar Mesa),
Near Riverhouse, south of Bluff, off the San Juan River (really fun pulling the trailer through, what the locals call, "sugar sand." (another great place to destroy a trailer tire...)
Spring Canyon river bottom; Green River
I will say one thing about that 228BH of mine. I has gone everywhere I needed to go, up and down ledges, on some extremely nasty roads, some with sharp rocks, and only problem I have had is the ST tires, and the having to replenish the battery fluid level after settling in and when getting back home.