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A DSL Internet phone line has a high frequency (internet high speed data) and a low frequency (voice & analog data), both can be used at the same time. Most consumer DSL services require filters to be added to every telephone extension in the home.

Any telephone device (fax modems, fax machines, telephones, answering machines, TTY etc.) require a DSL filter to block out the high frequency. The high frequency noise on the line causes these devices to “hear” lot of static/line noise from the high frequency, effectively causing faxes to almost always fail.

The DSL modem itself doesn’t need a filter, because it’s purpose is to use the high frequency signals on the DSL line. Filtering a DSL modem would cause performance issues, or make it not work at all. Some DSL modems may have a telephone extension where you can plug in a telephone directly, and you’re right, a filter usually isn’t needed for this.

If you’re plugging your fax modem into your DSL modem’s telephone extension , then I would suggest plugging it directly into a filter, and then another telephone jack in your home.