Phone voice is lagging

I am having an intermittent phone lag, wherein when we speak it takes as much a two seconds for our voice to get to our customers on the other end of the line. It will happen for a few hours, then stop and we are fine for a while. I have called the 800 number, and no one can help (the last guy hung up on me and never called back) and they even sent out a tech, who looked at our phone modem and said there was nothing wrong. This is my second dead end issue with Comcast. Support has utterly failed us at every turn. Anyone on here care?