Things to check with the modem are:

Make sure that the UART reads 16550

-  Try lowering your port speed to 57,600. 

-  Click on MY COMPUTER
                  DIAL UP NETWORKING

-  Highlight YOUR CONNECTION ICON (Internet dialup icon)
   Right click the mouse button so that a menu appears and click on properties.

Then set the maximum speed to 57,600. 

- Click on the CONNECTION tab and then the PORT SETTINGS button.

Lower the buffer speed settings to the lowest notch and uncheck USE FIFO BUFFERS
(since software based modems use a virtual UART this will not reduce its performance)

THEN HAVE THEM RETRY THE CONNECTION IF IT SOLVES THE PROBLEM, SLOWLY HAVE 
THEM INCREASE THESE SETTINGS TO THE HIGHEST CAPABLE. 
 

Also make sure that the UART chip is 16550 under the modem diagnostics.

Check to see if ERROR CORRECTION is enabled.

(Error correction was created to resolve performance problems with modem connections.  Without Error 
correction your modem would never connect or frequently disconnect.  It lowers and raises speeds
according to line conditions)

-  USE ERROR CONTROL, COMPRESS DATA, USE FLOW CONTROL 
    HARDWARE )RTS/CTS) are all checked
 

If all these are right then try inputting this AT command in the extra settings portion of the modems properties by going to:

MODEM icon found in CONTROL PANEL.

Click on PROPERTIES

Click on CONNECTION

Click on ADVANCED

Input the following into EXTRA SETTINGS

ATS10=250S38=240
or 
ATS10=40S25=10
Then try dialing your provider.

Go to the My Computer Icon and Right Click.  Select Properties.
Click on the Performance tab and Select the Graphics button.
Bring the Graphics Acceleration down to 1 knotch. (this brings down the Video Cards CPU usage to allow
the modem to use more)

Make sure the modem Enumerator IRQ is above 9

If this fails have them connect to the USR test line.  Just by them connecting proves that the modem
connects at a v90 standard.  This shows that their is something within the providers protocol which is
having compatibility problems..

-For the Softmodem an Intel 166mmx, AMD a 233, Cyrix a 266 with MMX are needed. 
 These are just the minimum the more ram and speed and L2 cache you have the better
  the response from the modem.