FAll (Fax All) is a mass faxing program that uses the old efax linux commands to send out a text-based message to any fax number in a csv, using a hardware fax modem.
With a VOIP line for cheap amount of money you can fax 5,000 faxes a day which would otherwise be cost prohibitative with cloud based fax service providers. Note that using the ObiTalk adapter with Google Voice, it is possible to fax free of charge, however mass faxing, what ever the intention is automatically flagged for spamming and goes against Google's policies. It is more reasonable to just pay $5/mo for a VOIP number for mass faxing.