Despite most ISP broadband setup instructions saying this there is still a misunderstanding somewhere when it comes to filters. So here is my Quick Tip attempt to help 
When you are setting up broadband in your home of office you will be given some micro-filters, the amount depends on how generous your chosen ISP are feeling!
The reason you need these filters is because when you have broadband activated on your line you effectively have two signals coming down that one physical line. Therefore you need to separate (filter) them. The filter sends the analogue signal to the socket on the filter to which you plug in the phone or fax or old style dial-up modem. The other small socket has just the digital signal that is required by your broadband modem.
All that said there is still one important point to remember. You MUST have a filter on any phone in the house or office no matter if there is a modem connect to that socket or not! The reason for this is that otherwise the phone will interfere with the digital signal that your broadband modem requires to connect you to the internet and thus your internet will randomly drop out!
For some that seems obvious but for others hopefully that will help
March 25th, 2008 at 7:52 am
That’s not strictly true. You only need a filter on wall sockets that are in use. If there is nothing at all plugged into the phone socket, then you dont need a filter.
Hope that clears things up.
Russ
March 25th, 2008 at 8:40 am
Hi Russ,
The post however is true/correct in the sense that I said phones MUST have filters on, rather than wall sockets 
Thanks for the comment. You are right indeed, there is no need to have them on unused wall sockets, I maybe should have stated that
Cheers,
Graham.