You can't assume that just because you connected at a speed like 48.3KBps that you will stay there. Today's modems automatically fall back to a lower speed if the line noise is too high to maintain a faster connection, but sometimes they fall back too soon or too far.
Here's how to do it:
Click Start the button.
Select Settings.
Click Control Panel.
Double-click on the Modems icon.
Select your modem.
Click the Properties button.
Click the Connections tab.
Click the Advanced button.
In the "Extra settings" field, type S36=7
Click OK to save your settings.
Source-
http://www.indiastudychannel.com/resources/95158-How-improve-your-dialup-modem-preformance.aspx
