How do I config "Roaming" Phones on Multisite UC500

Question

I have two UC520's in two geographically different locations with 7970G handsets configured.
Site 1: 192.168.10.xx
Site 2: 192.168.20.xx
Intersite dialing works just fine and Multisite is configured via CCA with status of UP.
I have one user who will be moving between sites for long periods of time (6 months at site 1 and the other 6 months at site 2). 
I want to the ability to phyiscally move his phone from site 1 and plug it into site 2 and have it retain all site 1 settings (understanding the limitations of having that phone statically register to site 1, call park, etc).
UC520's are currently configured for MultiSite full mesh however when I have tested plugging in a 7970 phone from site 1 into site 2's port(s) it does not register.
Under the dated Multisite deployment guide (https://supportforums.cisco.com/docs/DOC-9568) it states:
"Can I move my IP phone between multiple sites?
Yes. In this case the "roaming" IP phone will be statically configured to register to its home site even if it is physically moved to a remote site. The roaming IP phone will receive all its services, including PSTN & voicemail from its home site, essentially making it a Teleworker phone."
Does anyone have this configured and workding? Am I missing a configuration step outside of having Multisite configured between the two sites and simply unplugging a configured extension from Site 1 into Site 2?
Other than configuring the phones via CCA to their Home UC I cannot see how to "statically" configre them to register with the home site.
Appreciate any guideance.

Answer 1

Jordan,
Firstly, thanks for your continued replies, I really do appreciate them.
For what in theory seemed to be a simple configuration, this one has me pulling my hair a bit and of course it is for a single user!!!
I went with your initial recommendation of changing the Voice VLAN so that each site now has a unique Voice VLAN address range.  I then deleted all multisite configs and reconfigured them after the address change had been made.
Touble is I am not able to ping back an forth on the new Voice VLAN range and not quite sure where in the ACLs I need to modify or add the new Voice VLAN info (again all configuration has been done through CCA so it has made all updates)
I have now:
Site 1:
Data: 192.168.10.xx
Voice: 10.1.1.xx
Site 2:
Data: 192.168.20.xx
Voice: 10.1.2.xx
I am able to do intersite dialing between the two using (Intersite Digit)-(Site ID)- (Extension) and calling back and forth works fine.
When I move a phone that was configured to Site1 and plug it into a switch port in Site 2 it does not register
I update the TFTP to use address of 10.1.1.1 (Site 1's TFTP Server) it still does not sucessfully register.
I am able to ping:
Data - Site 1 - to - Site 2
          Site 2 - to - Site 1
(all good here)
Voice - Site 2 - to - Site 1 but not the other way around.
Obviously I need to include 10.1.2.xx in the range of IP's that can be seen throught he VPN however I do not know where to make those modifications. Do you have any suggestions on that or a good place where I can find that documented?
Also in this configuration should the "Teleworker Phone" option be checked for the extension that will be statically configured to it's home site?
Any direction would be greatly appreciated.