The problem here is the comcast service, which is a digital phone line, which don't work well (or often at all) with analog devices like fax machines, because of the compression and conversions that happen.
What you'll need to do to attempt to get around this is to make some adjustments to the settings on the machine.
1. Open One-Touch Speed Dial panel. 2. Press Data Registration. 3. Press Set. 4. Use N or M to select 3.TX SETTINGS, 4. Pick ECM TX, and set it to OFF. Press SET again to save and exit.
If you have any other questions, just let me know. Otherwise, please rate my service; hopefully I have earned 5 Stars. I also invite you to contact me directly with any future support needs at http://bit.ly/1QwLxGh
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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. Again I apologize there is not a better outcome here, though this is really beyond either of our control. 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.