Staticy or Robotic audio when using voicemail and using Mitel 5212

Wondering if anyone could point me in the correct direction to get this issue fixed. When leaving a voicemail or listening to voicemails using a Mitel 5212 only the audio quality starts out crisp but then quickly turns staticy, robotic or even over driven. I have tested the voicemail functionality on grandstream phones, analog phones and web UPC client, all sound great.