Plantronics Vista M22 Audio Processor Acoustic protection and compatibility with those telephones that do not allow direct connection of headsets. Plantronics SoundGuard Plus Ensures loud tones are quickly reduced to more comfortable levels and ensures that the normal, human voice remains natural and unchanged, for face-to-face clarity of communication. Plantronics Call Clarity technology uses electronic signal processing to reduce phone noise and maintain constant volume levels, as set by the user.