The broadband works on a different frequency than the phoneline, hence why you need the filters.
As has been suggested, try the phone direct into the socket.
If that works, try the phone with a filter into the socket.
If that doesn't work, and the socket is the main one, then its the phone or the socket.
Try a different phone - if you don't have one, could you borrow one from a neighbour.
Another option to confirm if the socket is working is test it with the sky box (assuming you have sky) - press services then go to settings and then move along the menus until you find signal - that will tell you if the phoneline is connected (assuming the box is connected to the phone line)
If that doesn't work its the filter - so try a different filter. Any socket that has something connected needs a filter.