Make a short location list
Include home, work, school, a regular commute and places where a call must be dependable. Note indoor and outdoor use. Terrain, construction materials and the distance from local infrastructure can affect reception.
Read the official map's legend and assumptions. Population coverage is not the same as geographic coverage. A colored area is not a guarantee of indoor performance or partner-network eligibility on every plan.
Run repeatable checks
Use the intended device and, where practical, the actual provider's line. Test a normal call, SMS, a web task and a sustained upload if that matters. Repeat at a busy time. Keep comparison tests close in time and place.
Record the task result, not just signal bars. Bars are a device presentation and cannot fairly rank different phones or providers by themselves. Wi-Fi calling may help indoors but depends on a working broadband connection and supported setup.
Interpret a failure narrowly
A problem in one basement does not prove a nationwide fault. A problem everywhere on one line may be device, account or SIM related. Several people failing in the same area gives a different clue, but still needs official confirmation.
Use official service-status tools for current incidents. This website has no live outage feed. If an essential call cannot connect, use another available means of communication rather than relying on a settings experiment.