Making a call or searching for something on the internet is something most who have lived in urban or suburban areas take for granted. Once you reach those wide open spaces in the West, neither internet nor cell service are guaranteed. We first noticed this in Iowa.
We avoided I-80, chosing to travel on 22 or 92. After visiting Des Moines we traveled for hours with no service. AT&T appear to have abandoned a large section of southwest Iowa. Nebraska had similar service.
We were becoming quite annoyed and even considered switching one of our phones to Verizon. We stopped at a store in Rapid City and were quickly dissuaded from that by the $100 per month fee.
Talking to other campers we found others had the same complaint with AT&T. People also had the same complaints with Sprint and T-mobile and even Verizon.
Yellowstone was the most disappointing because the cell service maps looked decent. The reality was that sometimes you had signal, sometimes not. And even when you had signal, the chances of actually making a call and staying connected were 50-50 at best.
So enjoy that 5 bar service while you can. If you have no service though you are likely outside with a great view.