What is the best cell phone provider for someone who does not need to use their cell phone alot?

We want to get my boyfriend’s Mom a cell phone for Mother’s Day. She needs a plan with good service coverage so she can travel with it but will not use the phone frequently.

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Which cell phone service provider will provide me the best service at the most nominal charge?
Cell Phone Forum: Choosing a Cell Phone Plan
Cell Phone Forum: Choosing a Cell Phone Plan