The short answer
Combine official maps, local experience, device compatibility, and a real-world trial instead of trusting one nationwide percentage.
Step by step
Check each carrier's map at home, work, school, frequent routes, and indoor locations that matter.
Confirm the exact phone supports the carrier's important bands and has passed its compatibility check.
Ask nearby users about calls and data at the same times and buildings you use.
Use a trial, temporary number, or one-line move to test calls, data, uploads, hotspot, and congestion.
Keep the old service until the test succeeds and you understand the new carrier's return and refund rules.
Before you finish
Keep screenshots, receipts, chat transcripts, confirmation numbers, and the first bill after a change. Carrier terms and account eligibility can change, so verify the final price and conditions directly before submitting an order.