Make sure all your phone lines are good. One by boring one. We thought we had bad numbers, but we had a bad line that stopped hundreds of faxes getting through. Luckily, Brooktrout's fax boards come with diagnostic software that allowed us to test lines. Once we tested the lines, we found the bad one.
Intermittent stuff is the worst.
Test before blasting. With the next three you make sure your fax blaster is handling all the possible types of numbers at the top of every fax blast list so they're ready to send as soon as you have your document ready. Harry is obsessive about fine-honing the look of faxes. He'll spend half an hour fiddling with one page. He thinks faxes are as important as magazine covers.