Are these two fax machines ext of a PBX? Some cut the signal too low to work. Other systems can reduce the data flow to the point it won't work. Are you actually routing thru the public network or a private net?
If you "hear" the fax tone, maybe the level isn't high enough to latch the connection.
If it works to any one else, it has to be something common to the two locations.

Ken
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