I have never used the VOIP, and I very rarely experience lag of more than 1 or 2 seconds (if the customer has slow/dsl-like speeds). That is over a sample of 200+ hours using teamviewer 4 and probably 30 or 40 hours of TV5.
Your speed isn't too bad, but the upload could be better. What was his speedtest? Did you try a pingtest?
My download used to be 20+ Mb/s, and my upload was closer to 1.2Mb/s. I am glad to have more upload now for w/e reason.
Click either image to go to their site and do a test.