VoIP static, interference and buzzing is usually associated with the introduction of a signal or voltage into the VoIP connection that is then converted into an annoying sound.  In many cases the problem exists within the devices that are connected to the ATA .  Cordless phones can be susceptible to interference from other wireless devices transmitting on the same frequency.  If you are experiencing this type of issue the first thing to do would be to disconnect everything from the phone port of the ATA . 

Cordless phones can generate static if the signal is weak.  Weak signals between the base station and handset can be caused by too great of a distance between the two or weak batteries in the handset.  Another possibility could be interference generated by another wireless device. Using an incorrect power supply that has a different voltage. Having distributed the VoIP though out the premise without disconnecting the incoming line at the demarc.

An alarm system not integrating correctly with the VoIP. A dial-up modem device that is trying to dial out through the VoIP line. A baby monitor.  These devices can broadcast on a similar channel to your phone.   Check the power supply and not only make sure that it appears correct (has the manufacturer's name on it), but is the correct voltage and amperage.  Linksys equipment has different power supplies for different routers. 

Check to make sure that if you have two pieces of equipment from the same manufacturer that you are using the correct one. 

• Alarm systems Separate caller IDs
• Faults with the wiring throughout the premise can also cause noise or buzzing throughout the lines. 
• Interference can be caused by one or more wires that may have come into contact with metal, (other than the connection terminals). Wires that could be loose at a connecting point. Check all wires and connections for secure connections.
• Dead Air: Can be the result of wires crossed or coming in contact with another pair.  (For example a red wire crossed with a green wire if 2-pair wire is being used.)
• If the buzzing still occurs after a corded test phone was connected directly to the voicebox, then look at these possibilities:
  Do you have the correct power supply that came with the device? Try plugging in the power supply into another power source.
• Check all filters & connections & try an isolation test.
• Make sure all phones are filtered.
• Check filter / splitter orientation & condition.
• Try a different telephone handset