Actually, I'm not positive I can give you the legal solution but I'll go ahead and point you in the right direction even though I still don't know if your sending unsolicited faxes or not.
The problem your seeing has nothing to do with WinFax, it's coming from your modem.
By law, modems must be able to temporarily blacklist numbers for a certain period of time after a certain number of failures.
I'm not sure how many failures it takes or how long the blacklist lasts, but thats why these numbers become "usable" again for you.
The intent of the law was to not tie up phone lines, especially regular phones because they could interfere with emergency services.
For instance if you mistakenly put in someones home phone number, a faxing software could call that number over and over and over again tying up that line. (And really bothering the people answering the phone).
If you were to look at the instructions or specs from the manufacturer of the modems chipset you should be able to find initialization commands that will show you the blacklist list and commands that can be used to clear the list.
Or even better, you should just remove the numbers that become blacklisted and assume they don't work or the owners do not want your faxes.
If your getting blacklisted numbers and your sure they are not failures, then your initialation string must be incorrect or not correctly resetting the modem between faxes. It's also possible something is wrong with your driver or modem setup.
I'm not sure of the legalites of adding the "clear blacklist" to the initialization string, I would assume it is not legal.
I'll send you some more info in a private message, I don't want to post any of the commands in pubic.
Click the envelope icon on the top of the page.
Dave