One minor irritation on the BT website is that on the Coverage Checker page there is no Network Status Checker, whereas there IS on the otherwise very similar page for EE.
https://www.bt.com/products/mobile/about/coverage-checker/
https://coverage.ee.co.uk/coverage/ee
Using the EE page, I was able to confirm there is a problem in my area and it accepted my (BT Mobile) number for the text updates option, which shows how integrated BT and EE are now.
Came here for solutions with similar problem/confusion too. Now I'm reminded of the line:
"Would that it were so simple!"