1.  Field of the Invention
The present invention generally relates to telephones, and more particularly to telephones having speakerphone functionality. 
2.  Background of the Invention
Mobile telephones often include a speakerphone mode in which users can use the telephones in a hands-free configuration without the use of headsets.  When a mobile telephone is in the speakerphone mode, the sensitivity of its input audio transducer is increased.  The output level of the telephone's output audio transducer also is increased.  Thus, a user still can communicate via the telephone even though the telephone may be placed a short distance away. 
To prevent outbound audio from the telephone's output transducer from feeding back into the telephone's input transducer, and thus and back to the original source of the audio, a voice activity detector (VAD) is typically used to mute the telephone's input audio when vocal patterns are being reproduced by the output audio transducer.  When vocal patterns are not present in the outbound audio, it is anticipated that the telephone user will begin speaking.  Accordingly, the VAD then unmutes the input audio and mutes and the outbound audio. 
Because muting and unmuting of the outbound audio depends on whether vocal patterns are present in the outbound audio, the VAD must be able to distinguish between vocal patterns and silent intervals contained therein when a person stops speaking and is waiting for a reply.  To do so, the VAD typically uses general statistics about vocal patterns, such as energy and periodicity.  Oftentimes the outbound audio contains music signals, for example when the telephone user is on hold.  Music signals are much more complex than vocal patterns and VADs usually have difficulty distinguishing the music signals from vocal patterns and silent intervals.  In consequence, the VADs intermittently mute and unmute the outbound audio, thus creating outbound audio having a very annoying choppy sound.