Network issue.
See the logs by troubleshooter.exe, under log files > AudioLog.txt
[06/21/2013 11:18:37:148] ERR: AUDIO: IsSideToneEnabled(): Device does not support sidetone.
[06/21/2013 11:38:31:772] ERR: AUDIO: SetSideToneState: Device does not support sidetone.
[06/21/2013 15:53:37:911] ERR: AUDIO: HandleIncorrectCodec: There is not enough network bandwidth available to sustain the VoIP call.
[06/21/2013 15:53:39:942] ERR: AUDIO: CheckSoundDevice: No sound device found on the system. Number of devices(in= 3, out= 0).
[06/21/2013 15:53:40:443] ERR: AUDIO: CheckSoundDevice: No sound device found on the system. Number of devices(in= 3, out= 0).
[06/21/2013 15:53:40:943] ERR: AUDIO: CheckSoundDevice: No sound device found on the system. Number of devices(in= 3, out= 0).
[06/21/2013 15:53:41:443] ERR: AUDIO: CheckSoundDevice: No sound device found on the system. Number of devices(in= 3, out= 0).
[06/21/2013 15:53:41:944] ERR: AUDIO: OpenPlaybackDevice: Opening playback device returned 0x2 (A device ID has been used that is out of range for your system.).
[06/21/2013 15:53:42:552] ERR: AUDIO: CheckSoundDevice: No sound device found on the system. Number of devices(in= 3, out= 0).
[06/21/2013 16:43:18:071] EMG: : Local Logging level is set to 'error'.
[06/21/2013 16:43:23:143] ERR: AUDIO: IsSideToneEnabled(): Device does not support sidetone.
Logs showing at the time of voice drop it is network issue.
