When logged in online I can see the breakdown of my bill by clicking on View Calls. Under the ? next to the words Number/Name it says:
A name will show here instead of a number if you've previously added the contact to your 'Address book'. You can add more contacts to your 'Address book' by clicking on the phone numbers.
I have also found this Help comment elsewhere:
How do I change numbers on my bill to names?
Your online bill can display calls by name rather than number, so it's easy to see who you've called. You just need to log in to My BT, click on the Address book tab, and then simply add the names and numbers of the people you call most often. They'll then show up on your bill by name.
Alternatively, click on the View bill button, choose What you used on the left-hand side, and then click on View all calls. If you then click on a number you'll get a pop-up window that lets you enter a name for it instead.
Sadly, there is no Address Book listed under My BT and, when I click on the number (to change it to a name) as suggested above, nothing happens.
Up until last Summer, I used to be able to click on the number and easily change it to a name for my elderly Mum - this was hugely helpful to her as she couldn't remember phone numbers so had the name there instead.
Can anyone help me to change the numbers to names on my bill? I think BT's own help pages need updating too!