It may stop altogether at lower speeds. Commonly called line noise, problems with downloading or the spurious characters or garbage that appear on the screen are usually from the central office of the phone company. It can make a big difference whether you call Out or someone calls In. Calling the phone company will rarely do any good, because the problem is not on your line. It is also much more likely to happen on local connections than long distance.
Older cities are more likely to have problems.
For more information on line noise and possible cures check the following web pages. ZOOM cannot and does not endorse or recommend any circuits on those pages. You use them at your own risk. I provided those Web addresses for informational purposes only. Don Hinds - Zoom Tech Ps.