There are many definitions for 'Virtual Phone Numbers'. The most common seems to be a phone number that forwards to an existing land line or existing cell (mobile) phone. Creating such a phone number is easy on the OnSIP platform.
First, purchase a new phone number under the "Resources" tab. This will be your 'virtual number'.
Next, add an External Phone Number under the "Resources" tab. This is the phone number that you want call forwarded to. For example, this number is your existing land line or existing cell (mobile) number.
Lastly, from the "Resources" tab, click on the "Phone Numbers" filter (little white boxes) to display only your phone numbers. Click on the virtual number and click "modify". Under the "Destination" setting, choose the External Phone Number you want calls forwarded to. Click "save" in the lower right corner.
Now, when anyone calls your 'virtual number' calls will be forwarded to your land line or cellular number.
Updated May 2015