When installing my U.S. Robotics modem earlier this year, I had some connection difficulties too. I don't know if this will help you at all, but you could try the following --
Go to Settings>Control Panel>Phone and Modem Options>Click the Modems Tab>Select Your Modem Model>Click the Advanced Tab>Enter in "+MS=V90" (no quotation marks)
Basicall you are setting your V92 modem into a V90. Might do something. May do nothing at all. If it doesn't help, just remove the the command you entered.