MacBook-Apple Modem use with DSL Modem

I have a MacBook and also purchased an Apple USB Modem. Have not installed it as I now have DSL and their modem. Don't appear to be able to fax with the DSL modem, thus want to know if it's a problem installing the Apple USB Modem for faxing purposes? Is it possible and do I need to disconnect the DSL modem to do so?

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