There is a flawed chip inside the batches of modems that made it early to the market place. It is possible that many of these modems may still be in a store inventory.
The modems connect...however as the telephone line conditions change the modem is suppose to adjust to it. All fast V.34 modems are suppose to adjust to line conditions on the fly by themselves. Something about this flawed chip makes it hang up instead.
Although you may have noticed this about USR modems there are also other brands that have this problem with earlier 28.8K batches.
Contact your 28.8K modem manufacturer if this is happening to you.