Possibly premium rate calls (not necessarily the rip-off 09 series) or texts to destinations such as 83111 (BBC Radio 3) or 61812 (Classic FM), that are not included in even the "unlimited calls" packages.
I wonder if the OP is not checking the detailed bill, in case someone in the household is making such calls without authority. Or possibly legitimate international calls that are not to listed countries.
Also I don't know the BT calls packages, but some providers' "unlimited" does not include normal calls to mobiles.
There is clearly something going on of which the OP is not aware.
Edit: A leaked/hacked router WiFi password is another possibility.
My broadband basic info/help site - www.robertos.me.uk.
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To argue with mindless bigots is foolish.
Edited by RobertoS (Mon 21-Sep-20 16:59:27)