If your printer is printing fine, but you get a "Low Toner" or "Toner Out" message, it likely is a false positive. It might be a glitch in the electronics of the machine or perhaps the manufacture programmed the machine to do this to increase its toner cartridge replacement sales. Replacing the toner cartridge will not erase the message. So, by all means, if you foolishly replace the toner cartridge, do NOT mail back in your old cartridge yet. Instead, go to The Google and put in something like "reset low toner message on [make/model" (without the quote marks). For some machines, it will be an easy fix by pushing a couple of buttons; for other machines, it might require covering the sensor hole to trick the machine into thinking it's full; for other machines (like the Brother MFC 7225N) there seems to be no solution (and don't bother calling Brother) except a factory reset (which you can get the steps for by putting into The Google "factory reset [make/model]" without the quote marks). In all events, do not start off your Monday morning dealing with this frustration, especially when you're also dealing with a refund AT&T owes you and failed to give you as promised and even failed to call you back as promised.
PS: After getting AT&T to give you the full refund without charging a restocking fee, and after taking a nice mid-day snooze, we escalated this "toner message" problem to our Level 3, which is the second highest level of our Department of Exceedingly Difficult Conundrums (EDC). Here's what to do to get rid of the "toner low" message: Open the front cover, push a button on the front (try one after another) until "Replace Drum?" comes up, ignore direction to press 1 for Yes or 2 for No, and instead press *00, and voila! If *00 does not work, try Start 1.