Try these solutions to improve OCR accuracy on fax images:
Ask senders to select Fine or Best Mode when they send you a fax. This produces a resolution at 200x200 dpi.
Ask senders to transmit files directly to your computer via fax modem if you both have one. You can save faxes as image files and then load them into OmniPage Pro.
Ask senders to use clean, original documents if possible. Sans serif fonts are easier to recognize than serif fonts.