There are many environmental parameters which can affect FAX over VoIP communications.
Although Voice over Internet Protocol (VoIP) is not a marketed feature of your Brother machine, there are several factors that affect the ability to send and receive faxes successfully when using this service.

1. Try adjusting the "Compatibility" setting on the Brother machine. Adjusting this setting lowers the baud rate (modem speed) and turns ECM (Error Correction Mode) Off when sending black and white faxes.
   *If your Brother machine has a touchscreen, please follow the steps of "For Touchscreen models" below.
   
   
   1. Press Menu or Menu/Set, and select Fax => Miscellaneous => Compatibility.
   2. Press the Up or Down arrow key to select Basic or Basic(for VoIP).
      Basic or Basic(for VoIP) reduces the modem speed to 9600 bps.
   3. Press OK or Menu/Set or Set.
   4. Press Stop/Exit.
   
   <For Touchscreen models>
   1. Press Menu and select Fax => Miscellaneous => Compatibility.
   2. Press Basic(for VoIP).
      Basic(for VoIP) reduces the modem speed to 9600 bps.
   3. Press Stop/Exit.
   
   
2. Verify that no other applications or computers are downloading files. While VoIP operates separately from the computer and they do not normally interfere with each other, the amount of data that can be transmitted from all devices on the system at one time is limited. If a FAX is being sent or received and a computer begins downloading, it will consume bandwidth and could prevent the FAX from going through.
   
   
3. Try sending the FAX again at an "off peak" time. FAX over VoIP is an internet application. If you have ever had a web site, which normally loads quickly, take a long time to load, then you have experienced network congestion. The same thing can happen to a FAX over VoIP. Data packets get delayed, but since there is a time sensitive machine on the other end of the communication the whole thing times out. Sending the FAX again later may avoid this.
   
   
4. Contact your VoIP provider and ask if they can optimize the network settings for FAX transmission.