Probably they are taking all circumstances into account and no saying what they may be.

Only thing that I can think of is if you are using a two line jack. It is possible that with the second line on the outer pair of the line cord there could be a crosstalk issue (either the fax interfering with the second line or vice versa) because these are not twisted pairs. Had this plenty of times with runs of old 4 wire station wire that is used for two lines. Not something that will usually cause the fax not to operate though I have heard of a case where crosstalk of the CNG tone caused a fax on another line to go into receive.

-Hal


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