SBS Fax problems

I have a SBS server, SBS SP1, Win 2k3 SP2 with a fax problem.   Faxes are frequently partially received and it takes several tries to send a fax.  The normal fax machine works fine on the same phone line.   I keep getting messages like:

"Reception error"      "Unknown fatal error"      "0x0000000c"      5/2/2008 14:49:06      5/2/2008 14:49:52      "U.S. Robotics 56K FAX EXT PnP"      "C:\Documents and Settings\All Users\Application Data\Microsoft\Windows NT\MSFax\Queue\1C8AC8532A8F2.tif"      0      "609-448-xxxx"      " "      ""      " "      0

in the activity logs.  I updated the modem firmware and switched to the USR provided driver.  This did not help.  I disabled compression and reduced the port speed to 9600 baud which also did not help.  The modem is a USR5686E.

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