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How many ADSL filters do I need?
You will need to fit an ADSL filter to every telephone outlet where a telephone is connected. Whilst there is no practical limit to the number of filters that can be connected to a phone line, we recommend a maximum of four filters and that the phone should be connected to a filtered sockets only unless the phone's socket has built in filter components. It is possible to attach a phone-doubler adapter to a filter in order to attach two telephone handsets. However it is important to note that not filtering one telephone device will affect other devices and this may stop all the other filters working correctly.