BT are his service provider, on caller display on his phone
Then I would suspect that Caller Display is not working, and call blocking is still turned on. See page 27 on the online manual. Does his other phone have Caller Display, and is the incoming number showing?
It works at your house because Caller Display is most likely working at your house.
Thinking about it, on the offending phone a call is displayed as an incoming call and you can still pick the call up
Then Caller Display is not working on their line, as it should display the caller`s number.
Is the facility switched on?
If you've ordered Caller Display check it's switched on by entering the code * # 234 #.
If it's active you'll get the message Caller identity will be displayed. If the facility isn't active then activate it using the code * 234 #.