By default, callers are directed to voicemail when you're line is busy. However, if you want calls to forward to another phone number instead of going to voicemail when your line is busy, you can turn on the Call Forwarding Busy feature.
To turn on Call Forwarding Busy:
Open your browser or Microsoft Outlook with the Enterprise Assistant Toolbar.
The Services window opens with the toolbar features, displaying the last feature opened.
Under the Incoming Calls menu, click Call Forwarding Busy.
Click the On radio button.
In the Calls Forward to field, type the number to forward calls to when your line is busy.
Do one of the following:
If you want to make additional changes, click Apply,then click on the next feature to set.
If you're done, click OK.
When you want your calls to go to voicemail again when your line is busy, turn off the Call Forwarding Busy feature by clicking the CallForward Busy icon (the red silhouette with the white arrow) on the toolbar.