VoIP: simple proof of IP network's quality

Suppose that some remote colleague were complaining about the application performance, and she were assuming "IP network performance" is to blame.

If I could call back by phone and explain to her: "I am calling over the same IP network. What do you think about the voice quality right now?!"

Isn't this quite a simple proof of IP network's quality! I now have a good purpose to implement telephony services on IP networks!