henrymat,
I don't think anyone has ever asked me that before; hmm. So, if you know the user is 1200 why send a call to 1600; would be my first question? If the user changes daily or frequently enough that they don't know who is there then how do they know if anyone is at 1600? I can't think of a scenario where I expect someone to be at a phone and not know what the extension of that person is.
In reality the system isn't usually designed to send calls to a Hot Desk Base phone as it is supposed to have someone logged on for it to be used. What you could try is forwarding 1600 to 1200 prior to logging in with 1200; not sure if that would work but just remember to remove the forward after logging out. Alternately, you could setup Dynamic Extension Express on 1600 and make 1200 the second desk phone for the time period they are logged on to it.
Oh, here is one I just thought of if you have an ACD License on your system. You could create a Hunt Group for each of your base phones you want to do this with; keep in mind you can only have 75 Hunt Groups total so this may not work for you. Then have all of your Hot Desk users in each Hunt Group and when they log onto a particular Hot Desk Base they pick the corresponding ACD Hunt Group to log on to as well. Man, that is a convoluted way of making something work, but when customers ask for odd things you get odd solutions. Then when calls are being transferred to that base you really transfer them to the ACD Hunt Group instead; of course the call won't go to the Logged In users mailbox as they aren't really being rang directly.
Thanks,
TE