Most maps model outdoor signal at street level and say nothing about conditions inside buildings. Passing through walls costs signal strength according to the materials involved, so a room deep inside a floor plan or below ground can be unusable while you are standing in the middle of the darkest shading. Step outside or go up a floor and test again to confirm that is what is happening.

Trust it at the level of regions — comparing one area against another — and much less at the level of a building or a single spot. The map is the output of a model fed with terrain data and mast positions, not a set of measurements taken everywhere, and operators generally label them as estimates for exactly that reason.

The one belonging to the partner network the plan uses in that country, because travel plans generally run over local networks rather than operating their own. If the plan's detail page names those networks, read their maps along your route. If it does not, ask the provider before buying.

Because maps and signal bars describe coverage, not capacity. When many people use the same network at the same moment, the shared resource available to each of them falls. It is not a fault in the profile or the handset and usually improves as the crowd thins. If you need to meet someone at such an event, agree a meeting point beforehand.

Download the map area while you are still on Wi-Fi, save the accommodation address as an image or note on the device, keep booking documents accessible without a connection, and write down an alternative route or a meeting point. A few minutes of preparation turns a gap in service into a minor inconvenience rather than a problem.

Read them together. An operator map covers everywhere evenly because it is calculated; a crowd-sourced map reflects real measurements but only where people carrying devices have been. Where the two agree, confidence rises. Where there is no crowd-sourced data at all, that is not evidence of an absent signal.