IPInstaller,
You can send CLI through to the DECT Phones using Analog ports and as for transferring calls between the two systems it will be easier to do from the 5000 to the Aastra then from the Aastra to the 5000, but it is possible.
From the 5000 side the Aastra will look like an analog port and from the Aastra side the 5000 will look like a trunk. We do this alot and if you want to make it really nice you will have analog ports and trunk ports going both ways, which is how we set them up if the customer doesn't want to think about how to transfer calls.
If you can figure out how to get SIP devices, not trunks, set up between the two then the problem becomes much easier, but we have not found a system that supports that setup yet; although there may be one out there if you look hard enough. That way both sides see the other as a SIP phone and can transfer calls back and forth along with Caller ID.
Thanks,
TE