Apart from the low-ish cap balance (and this will not be the cause of what your other thread mentions, there would look to be naff all wrong with that line, though it's not presented in the normal format I see.
Further to what others have told you, the issue is for Virgin to pursue and resolve on your sisters behalf.
Oh, and theres no 'if' about it, if you insist on BT coming to check the voice side of things, and nothings wrong, you WILL get a hefty bill.