You can assign a Better Voice virtual phone number to your Direct Line or any of your Extensions. You can do this two ways:
On the extension page, under the Phone Numbers section, click on the "Choose Existing" button to assign an existing virtual number in your account. Or, you can choose to "Purchase New Number" as well.
You can also assign phone numbers to your Extensions from the "Phone Numbers" page in the app. This provides a list of all of the virtual numbers in your account. Use the dropdown form option to select which Extension, or user, a phone number should be assigned to.