Microfilters are used to prevent ordinary telephone devices – such as phones, answering machines and fax machines – from interfering with your ADSL service. If your phone line is being used with any equipment other than your ADSL modem, you will need to use one microfilter for each phone device. Each microfilter is connected in-line with your telephone or fax machine so that all signals pass through it. Telephones and/or facsimiles in other rooms that are using the same extension will also require microfilters.