system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can every cell phone carrier claim that they have the nation's best coverage?

I see ads for both big and small carriers saying that they have the largest coverage and are most dependable in the U.S.  Are these claims actually investigated?
gpt
The way that networks test for reliability can be different. These tests depend on the phone used, the places tested, the method of testing, etc.

After all that, you still have the subjectivity inherent in the word "best." How do you define the most reliable network? Is it the least number of calls dropped in a specific area? Is it the area of their coverage? Is it the strength of their coverage in a city?

All major carriers have a test that considers them "best." The claims are worded such that they're legally ambiguous and you'd have trouble claiming that they're not the "best" by some metric.