I had that same scenario occur at one of my customers out of the blue. Messed with it and messed with it for six weeks between SBC & CBeyond. Have seven other fax machines working at the main location with no problems on a FXII using IST ports thru a T1-PRI. Finally put that problem fax on a POTS line and haven't heard anymore. Moved the fax to another port, had SBC bring in a new loop, put the fax straight out of the switch, tried four other fax machines of different brand. This definitely whooped me and that doesn't happen too often.