Softphones Best how too

Ok, coming from the broad-soft world it was quite easy to have a user register multiple devices to his account like a phone in office, a phone at home and a softphone and you use the shared lines. Fast forward i want to be able to allow a user via physical phone to also register a softphone which would allow him to answer calls if he is out of office etc or make calls. Thirdlane does not support multiple devices to register for one extension. So after reading some posts i tried to create a special line for the softphone but for some reason it comes back as not authorized. So if i go and create a new user extension i can register the soft phone just fine. So can a special line be used ? What is the best way to handle soft phones as a 2nd extension for user ?