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When hooking it up, be sure that the phone cord from your wall jack, or from your digital phone service box, goes to the 'LINE' jack, not the 'PHONE' (or, phone-handset-logo) jack.
Do you have digital phone service? if so, there may be, and often are, problems with digital phone service, which was not meant to have faxes sent over it, and often does not support faxing... depending upon which exact digital service you have. Also, the faxing machine may have to be adjusted to a lower Tx/Rx speed, and/or have ECM disabled in its settings. Let me know how it goes, and what phone service you have, and I will advise you further. Thanks.