I have accomplished much of what you look for by using the AgentCallbackLogin feature. With the following .conf entries a roaming user can sit down at any physical extension, log in, and make and take calls with his own number.

In my implementation I have assigned all of these roamers extensions in the 20xx range. Extension 2199 is used to log in and extension 2198 is used for logging out. My physical extension numbers happen to be in the 10xx range, but that's irrelevant to this function.

First, in agents.conf, we define these roaming employees as Call Center Agents:

updatecdr=yes ; This uses the agent's ID instead of the physical extension number in the CDR.

agent => 2001,1234,Roger Ramjet ; UserID,Password,Name
agent => 2002,1234,Jonas Grumby
agent => 2003,1234,Ned Flanders


Then, in extensions.conf, in the same context as all ordinary extensions, we put:

exten => _20XX,hint,Agent/$(EXTEN)
exten => _20XX,1,Dial(Agent/${EXTEN},300,t)
exten => _20XX,1,Dial(${HINT},20,rtT)
exten => _20XX,2,Congestion

exten => 2199,1,AgentCallbackLogin(|${CALLERIDNUM})
exten => 2198,1,AgentCallbackLogin(|#)

Now, in order to log in, the user does not need to know the physical extension number of the desk he is sitting at. $[CALLERIDNUM} takes care of that. Logging out requires calling 2198 and entering a blank extension number (#) when prompted.

As long as the user is logged in, calls to his 20xx number will be presented to that physical extension. Voice mail captures messages regardless of whether the user is logged in anywhere or not.

I have not yet worked out how to change the COS on the physical extension when a real user is logged in, so further contribution would be appreciated.