I think it only appears if the Obi is the router. In my case I have a 202 connected directly to the modem and it shows my external IP address.
The 110 is behind that 202 and does not show the external IP address.
Is the reason that you cannot use CallCentric on some of the units because you are out of SP slots? I don't know if using a Gateway would help. I believe that you want to know the last IP assigned, and if the box needs a reboot, it may not be trying to send its IP anywhere.
Also, perhaps a Syslog account somewhere, logging just registrations.
Essentuually a Dynamic DNS system for an Obi, whether it is a router or not.