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.
Then 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.
Type in the modem string in the Extra initialization commands field.
Click the OK button.
Close out of all other windows.