Right click on the My Computer icon, located near the upper-right-hand corner of your desktop.
In the drop-down menu, click on Properties.

Click on the Hardware tab.
Click on the Device Manager button.

Click on the arrow left of Modems
Double click on the listed modem.
In this case, I'll be using the USB Data Fax Voice Modem.

Click on the Advance tab.
Click on the Change Default Preferences... button.

Go to Compression, drop the list down and select Disabled.
Click the OK button.
Click OK and close out of all the other windows.