Poor/unusable signal strength.

[Jred55]

Then the coverage checker is pointless unless it accurately reflects the reality of coverage. I took out a contract on the basis of the coverage (4g excellent indoors and out) and it blatantly isn’t true. I get one bar at best.

Worst case it’s being mis-sold, best case it’s inept. 

[bristolian]

The coverage checker is a computer-based prediction of expected service, but as with all mapping software is subject to numerous variables.

No network will guarantee coverage indoors, but if you have good service outdoors then WiFi-calling is an excellent mitigation.

You can also use the "check status" option on the website tool to identify any known faults that may be causing degraded service.