Remove any previous installations of the ISDN Pro TA. This can be accomplished two ways.
Either through the Phone and Modem Options in Control Panel:

Or through the Add/Remove Hardware Wizard in Control Panel:

If the modem is not present, proceed to the installation steps below.

Complete steps 1 through 4 from the Install Guide provided with the TA. Turn on your computer.

In USB Mode the following screens will appear:

Click Next. You will see:

Select Search for a suitable driver for my device. Click Next.

Select Specify a location. Click Next.

Browse to the directory where you downloaded the file usrprota.inf. Click OK.
This example shows that the file is located on the root of the computer's D: drive.

Click Next. A Digital Signature Not Found message will appear.

Click Yes.

Click Finish to complete installation.

To verify installation and configure the TA, click Start, select Settings, and click Control Panel.
Double-click the Phone and Modem Options icon.
Click the Modems tab.

Double-click 3Com USR ISDN Pro TA. The following message will appear:

Click OK.

Click the Diagnostics tab.

Click Query Modem.

Your TA's command set should appear in the box.

Click the ISDN tab.

Type your Directory numbers and SPIDs specified by your phone company.

Click the Advanced tab.

Click Change Default Preferences.

The General tab allows you to modify Port Speed, Data Protocol, Compression, and Flow Control. Click the Advanced tab. 

The Advanced tab allows you to modify Hardware Settings.

Click OK to close Default Preferences.
Click OK to close 3Com USR ISDN Pro TA Properties.
Click OK to close Phone and Modem Options.

 Verification and Configuration are complete.

The first time you use the TA with Dial Up networking, an error message will appear.

The message appears because Windows is configuring the TA with the Directory Number and SPID information provided above.
Wait approximately 15 seconds for the TA to reset, then click Redial to establish a connection.