Logging into user and changing extension

We currently use Televantage. In Televantage each device has an extension and each user has an extension. When the user logs in with credentials they also enter the device ID. Once they are logged in the station IDs extension is updated to that of the user. The users sit at different stations each day so their station ID changes each day.

In setting up Freepbx I have found lots of information on how to map a static extension to a user. However, how do you set up a user so that the extension changes depending on where they sit and how do they login with that?

Forgive me if this is answered elsewhere

The only supported way is to use the Login REST Phone App. The unsupported way is to set up FreePBX to use User and Device mode.