We have 2 SBX customers that use Auto Fax with similar problems. Intermittently they are on a call and suddenly a fax-like tone (I have never heard it) breaks into their conversation. They can no longer hear each other. I jury-rigged a single line telephone ahead of the SBX on a 3-way switch. When the problem occurs the customer switches the C.O. off of the SBX onto the single line - problem gone. Seems to indicate tone coming from within SBX. One system was 3.5 and I upgraded to 4.0Ac - no change. Has Charter lines. New customer with 4.0Af similar problem. ATT pots lines. Both sites use Auto fax. Thx.