£20 worth of tools and a few hours of asking questions here and getting advice will sort this out for you.
If you've wired a plug you can fix this.
Start by taking a look here,
(first half of page is most relevant,, and I would only connect 2 and 5 internally these days.)
Other thing to do is carefully remove each phone extension from the wall, take a photo of the rear of each and post links to them here.
Are your extensions DIY kits using "flat" cable?
Finally, are you CERTAIN that evey item in the house connected to the line is filtered? Alarm? Oil or gas tank dialler? Sky box? That never-used phone in the spare bedroom?