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If your telephone line is ADSL enabled, the ADSL signal is applied over the same wires from the exchange as your normal voice calls - both services can operate simultaneously. Although the ADSL signal is generally outside the range of a normal audio signal, you would still be able to hear some interference from the ADSL signal on your voice call, so you will need to fit 'microfilters' (also known as ADSL splitters) to your phone extensions. These provide a filtered phone outlet, so that the ADSL signal is removed so that you can use your phone normally. You would normally need a microfilter on all telephone sockets that are used for telephones (unless you have fitted a 'master' microfilter adaptor to your BT master socket, but that is rare nowadays).
Draytek supplies two types of Microfilter; a compact one and a 'tailed' one. As well as your personal preference to the style, the 'tailed' model provides a more comprehensive filtering method (sometimes known as 'active' filtering). Both types have a socket for the telephone, and another socket (RJ11 type) for your ADSL device (modem/router).