If I read you correctly, you're using a different internal modem in each computer. If that's the case, I would guess that the different modems are the cause of the varied performance. The term "56K modem" is not very definitive. After the 14.4, 28.8, and 33.6 standards, the "56K" era started. Certain companies seemed to compete and there were various and different operating standards for modems. There was no single accepted standard. Because of that some modems wouldn't "talk" very well to other modems. After years of inconvenience to the consumer, the V.90, later V.92, standard came about and most modem problems went away. If you have any modems older than V.90, replace them and you should see better results. But if any modem is working to your satisfaction, leave it alone.
 
There have been inexpensive internal "WinModems" on the market, which used the resources of a (Windows) computer to work. They were not highly regarded. I would avoid Windows modems or WinModems that require Windows to operate. Look on the box to see if it requires Windows. Generally preferred are freestanding "hardware" modems which will operate on most any computer and OS, functioning independently of the computer.

I started using US Robotics 5686 external modems when the V.90 standard became common. I still use them when I connect by phone line. They work very well and are available on Ebay for very little money. Similar models are available new. The externals are kinda bulky but can be switched from one computer to another with a serial port switch if that helps. There are probably good "hardware" type internal modems available. I don't know about them because I've always used externals. Maybe somebody else would suggest some good V.90 or newer internals to you (and me).

Another issue you may have is incorrect reporting of modem speed. Many modems did not report the true connect speed. For some reason, they would always report a higher number, never lower. I think most can be made to report properly by upgrading drivers and/or using modem "strings" which are commands to the modem. I think strings became pretty much unnecessary after V.90 came into use. The most up to date drivers may help, but again, if a modem is working good, don't fool with it.

The one that won't connect at all needs special troubleshooting. Has it been "fried" by lightning? Is it seated well? Does it have a good IRQ? Have a driver? See what Windows "Device Manager" has to say about that modem. Etc,etc.

Hope this helps. 
Try here for lots and lots of good connect info:
http://modemsite.com/56k/trouble.asp



>From: Sue Rose <[log in to unmask]>
>Sent: Dec 31, 2006 10:07 PM
>To: [log in to unmask]
>Subject: [PCBUILD] internal 56k modem
>
>Two identical gateway g6-400 towers--one running win 98se the other running win 2000,
>Aptiva IBM running win 98 first edition, custom built tower running win 2000
>All computers are around 1996-1997
>The gateway running win 98se tower nornmally connects via the modem to this my account (edu) at 44, 000 or 45,333
> the aptiva was connecting at 48000
>the gateway tower running win 2000 was at 28.8kbs
>and the custom built tower never was able to connect
>
>Why this disparity--same phone line that i plugged into each tower

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