This is a question we get asked a lot. The most possible reason is the modem is opened by other applications, such as: Windows Fax Service. When FaxMind trying to open the modem, it got an Unable to open this fax device error message.
Please close/turn off or uninstall that application/windows service ( i.e. Fax Service), then reboot your computer if it's neccessary, and try again.
Here is a part of the log file:
13-07-01 23:42:53.459 [SYS] Outbound fax service started...
13-07-01 23:42:54.959 [SYS] Inbound fax service started...
13-07-01 23:43:57.146 [DIVA] Initialize Diva API failed.
13-07-01 23:43:57.287 [COM4] Unable to open this fax device.