I've had this problem over a year ago. It was really annoying, and since I use VOIPo for working from home, it made me late to a number of meetings. At first Tech Support updated my "DTMF method" and moved me to a closer server. Those seemed to help, but the problem returned the next day. Then they updated my "jitter buffer" and the problem went away completely.

I looked up "jitter" in Wikipedia (http://en.wikipedia.org/wiki/Jitter), and that sure sounds like it would be related to the problem. So I would suggest, from my experience, that you ask about it.