Switchvox - Is it Possible to have Multiple Phones Registered to the Same Extension?

There is no way to register a phone to an extension that already has another phone assigned to it. Switchvox only allows 1 phone per license, which also counts for softphones.
In the new version 6.6 and after  you are allowed to add additional phones extension to a main extension, you will still need to create a extension but it will not require a use of a license. 

Call rules of an extension can be manipulated to send the call to the user's desk phone and another phone extension at the same time and you can control this feature with time frames or with user status, but not multiple phones to the same extension.