AT&T and Verizon Coverage in Yellowstone

I was in Yellowstone last week with my family, my adult neice and
nephew's family, and my sister-in-law's family. 7 Verizon network phones
(5 Verizon, 2 PagePlus) and 2 AT&T phones.

Except for the Old Faithful Area, the AT&T phones lacked coverage in
most parts of the park. The Verizon network phones had coverage
throughout the park, though in the Fishing Bridge area the coverage was
marginal at 0-1 bars of signal. I had AMPS coverage in many areas where
there was no digital coverage at all. However in some instances, even
with a strong AMPS signal roaming off of Verizon, I got a strange
recording about how this feature was not available (on the Verizon
phones) and on PagePlus AMPS it wanted double-dialing, indicating that
I'd be paying roaming charges. Since I'm still on the old America's
Choice plan, which is supposed to allow off-extended-network roaming at
extra charge, I don't know why I couldn't use an available AMPS network,
though it did work on occasion (it may have been two different AMPS
networks, I don't know who the carrier's were).

In any case, as I've found in all other national parks I've been to
recently (Yosemite, Lassen, Everglades, Crater Lake, Redwood), if you
have AT&T service and want cellular coverage when traveling, you should
purchase a tri-band Verizon phone and activate it on PagePlus. AT&T's
GSM coverage is just not anywhere near the level of CDMA and AMPS
coverage once you're out of the urban areas.