I have seen this happen at various times in the past, usualy its when downloading larger attachments. This can happen for any number of reasons, one being a degrigation in line quality. The modem will try and negotiate a lower connection speed but sometimes it has issues re-communicating. This is really where the Industry streghth modems come in vs. external or internal in expensive modems. Honestly, not all modems are created equal. If your hanging a couple of 50.00 or 100.00 modems, its very possible you have found the limits, you have two inexpensive modems at either end. Modems that are created for pools often handle issues better then what you might have. I might suggest that a better solution for your sales group is to use Dial in accounts from your ISP, using their connection points and have them VPN into your network.
It can also be related to the line the sales people are using or the line that supplies your modem or pool. If your using regular POTS lines, most Bells will only guarentee 14.4 or 19.2 connections, basicaly fax speeds. You might have to have a conditioned line for this use, costs a bit more but better quality. Finaly You might hook up a regular telephone to the line and make a few calls and see if your getting static on the line, static that you can hear. Since analog modems are Audio based, if you do hear crackling on the line, it would cause issues. If you do, and the Telco can verify it, they will fix audable issues. I hope this helped answer some of your questions.
Pete
