Phone numbers allow callers from any phone to be able to call in. Phone numbers can be attached to users, groups, apps and resources (except to another inbound phone number). When you create a phone number, you can select the area code and region. The phone number then appears in the selector table below.
Phone numbers need to be assigned. Clicking on modify allows you to choose which user, application, group or resource you want the phone number to be attached to. You can also assign a phone number from within the "Extensions & Phone Numbers" detail panes of any user, application, group or resource.