GXP-2000 hotdesk how?

Not exactly an Asterisk problem, but I’m hoping some people have run into a similar situation and can help.

I’m trying to setup a hotdesk scenario and am having an issue with how to setup the Grandstream GXP-2000 series phones. I can assign a phone to a SIP account on my Asterisk server, but cannot figure out how to set up the phone to allow a user to login to a phone dynamically.

I have a config setup whereby users should be able to login by dialling *11 and *12 to login/logout, but I receive an Asterisk debug message that the phone is fixed to an account when I dial *11 and have configure a fixed account on the phone (sounds reasonable). Yet if I remove all accounts from the phone, I cannot reach the server to login and no debug messages are received.

Can someone point me in the right direction to allow users to login dynamically in a hotdesk type situation using this hardware? Or at least tell me what info to pass to to assist in finding the solution?

Many thanks.