Ok. Try this :-
1. Pull out all phones and other divices from all phone line sockets in your house.
2. Plug a splitter/filter into your main socket and connect the broadband to the DSL/Broadband Port.
3. Connect the other end to your broadband router.
4. Reset your router to factory settings as per the instruction manual.
5. See if broadband works.
If that doesn't work, call PlusNet on 0845 1400200 and select option 3 for technical connection questions and they will give you more info and maybe submit a ticket on your behalf to BT.