Hopefully I can offer some insight.. I have a similar set up (except with the GigasetOne [Same Device, different name]).. Here's what I did (pretty much what Purplelaser suggests).
Cellphone <----> GigasetOne <----> Obi 110 Line Port <----> Obi SP1 <----> Asterisk
While I'm not using the feature codes to select the different lines, I use Asterisk to do the routing.
When I get an incoming call through the GigasetOne, it's passed to the Obi 110 who then passes it to the Asterisk box. With this set up the CID is preserved, and with the option enabled, it set's the name portion to "GigasetOne".