Hot desking and twinning are two different things. Hotdesk is allowing a user to log on any free phone and their personality is downloaded to the phone i.e their key appearances and any speed dials they setup. Twinning is having multiple devices ring when a prime number is called. This is accomplished through a personal ring group. The ring group contains the extensions that will ring when the main number of the group is dialed. If you want a cell phone included you need an External Hot Desk User license.
Anyway to the question any set which is setup to accept a hotdesk user will have a "login" button. You push that, enter your extension number and then a pin number ( for security ). Once that is done the phone will be "yours". If your number is also the main number of the ring group then all members will ring when someone calls you.
Note if you are logged out then calls typically will follow your forwarding or call rerouting.
