One way to tell which you have is from reading the box or supplied manuals. Another is by issuing ATI3 to the modem while in a terminal program. As a last resort to the very picky nature of ZOOM modems, when all else fails, and you think it is defective, try this in your init string. Yeah, I know. It disables compression.
Most files are already compressed anyway.
But this is the only way I can get mine to stay online. Think I've got a solution for you to post. Zoom has finally submitted a string that seems to work. All rights reserved.