There's a map showing which tunnel sections are now covered at
https://content.tfl.gov.uk/tube-map-below-ground-4g-and-5g-c... - but it should be rolled out across the entire network by the end of the year.
It's noticeable that it remains rare to actually make voice calls. I suspect that's down to the ambient noise levels rather than the innate politeness of Londoners, though!