Are you using DSL connection or Dial-up?
What OS are you using?
The fax line and voice line are using the same frequency so you should be able to fax directly from your pc fax modem when using Dial up connection but in DSL the data and voice are using different frequency. So in normal situations you wont be able to fax using DSL modem unless you use a separate fax machine.
However, if specified during the set-up of the DSL line, the signal can be separated so that some of the bandwidth is used to transmit an analog signal (for voice and fax) while the remaining bandwidth is used to transmit digital data. This allows for the use of the telephone or fax, and computer on the same line and at the same time. When inquiring about DSL with the service provider, specify the desire to use the phone line for voice or fax calls while connected through the computer to the Internet.
Reference the following setup options for using a DSL line with HP all-in-one or fax products.
Please refer to my solutions to your issue at http://www.fixya.com/support/t1440518-want_able_fax_from_computer_even_though
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