I've got a brand-new external USRobotics 56k fax modem attached via serial cable to COM1 on an HP DL380 running vSphere 4.1.
The guest operating system in question in Server 2003 Std, SP2.
Serial passthrough is configured properly, and incoming faxes through the Windows fax server are OK. Outgoing faxes, however, have problems. Dialing and connection seem to happen normally, but the status hangs on 'sending page 1' for about 2 minutes before complaining that the other fax didn't answer in time. The printed output on the receiving end has a lot of bad lines- graphics, especially, come out blurry or with random pixels where the graphics should be. This behavior is not exclusive to images, though.
I've lowered the port speed to 9600 with no positive results; reinstalled modem drivers; purchased a new modem; rebooted the host server, and guest OS. The only obvious thing I haven't tried yet is a new serial cable, but since I'll have to go onsite to install it I was hoping to get more suggestions I can act on while I'm out there.