Today I got the 'toner empty' messages from my Brother MFC-7440N printer. I printed 850 pages total and this is a TN-2110 which came with the printer. About 10 pages before I got the 'toner almost empty' (all in german so these messages might not be accurate) message.
The MFC-7440N would stop printing any pages. The last pages before this came out 100% fine so I wanted just to continue to print with this toner cartridge. A lot of googling lead me to several 'solutions' which did not work for me. I am not yet 100% sure what helped, but taking out the toner cartridge with the black case, shaking the toner, and putting yellowish paper tape across both holes on the left and the right of the black toner cartridge housing solved the problem.
After putting the toner back I could print a page normally and I no longer see the 'toner empty' nor the toner almost empty messages. The page count is still at 851.
I also found these hints: Open the front cover, then press the Back (german:Storno) button (the left bottom one of the four round black buttons), then press * 0 0, or * 1 0. But both did not work for me. (Do not press 1 or 2 as indicated in the display. This is for the drum.)