With CUCM 6 you won't be able to use the Cisco Jabber client. So, you will need to look at a 3rd party SIP softphone. I have tested a few softphones on Android and iOS. However, I have not tested integration with CUCM 6. Getting a registration is totally possible and the method should be the same. I just can't comment on stability.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...