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It may be receiving into memory. The most common cause (but not invariably the cause) is inability to print out the received fax. Test that ability to print out, by running a photocopy of a one page original.
Then, it may be (even if the photocopy comes out OK) that the Receive Mode is set to Memory Receive or similarly.
And lastly, it might be that the Receive mode is set properly, but that it isn't working.
Please tell me whether there are any other devices on the same phone line as the Brother machine's fax line, and if so, what devices ?