The issue is that she probably has a real phone line at work. What you have is a digital line through the cable company, which doesn't work well (or often at all) with faxes, which are analog and have to be converted back and forth twice or more send.
Chances are the people you're having trouble with are on a digital line too, compounding the problem.
Go into the fax settings menu and disable error correction and set the modem speed (tx speed) to the lowest possible option. These 2 adjustments should allow it to work even with the poor digital line conditions.
I am an independent technician and I've spent my personal time, unpaid, in an effort to assist with the question for which you solicited my services. This site works on the honor system, and to that end, I have prioritized working with you ahead of other paying customers.
Although you’ve already paid your fee to the site, I am only compensated for my efforts when customers such as yourself leave positive feedback for my work. I do hope you’ll consider leaving a 5 star rating for me, rather than “shoot the messenger”, so that I may get credit for my time spent in good faith to answer your question accurately.