Recognizing and Categorizing Symptoms of Voice Quality Problems

http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/30141-symptoms.html

http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/20371-troubleshoot-qos-voice.html

http://www.voipmechanic.com/phoneandvoicequality.htm

http://voip.about.com/od/voipbasics/a/factorsquality.htm

http://www.voiptroubleshooter.com/problems/robotic.html

https://sites.google.com/site/cicocm8/courses/voice-quality

https://wiki.voip.ms/article/Call_quality_issues