Doing volunteer canvassing for [name redacted] in rural areas of Pennsylvania, North Carolina and West Virginia, imagine my discomfiture when my T-Mobile coverage died, especially when others using AT&T continued to "get more bars in more places."
My solution was to go into an AT&T store and pay $15 for a GoPhone sim card kit, which included a $10 credit. Of course, this is for voice only.
For use as an occasional backup, I will be refilling this card for $25 every 90 days, rolling over unused minutes. See plan details here