There is a minimum version of Unity Express and IOS. Per the 6.1 guide, "The most likely deployment model to use Cisco Unity Express is the multisite WAN model with centralized call processing, where Cisco Unity Express provides distributed voicemail at the smaller remote offices and a central Cisco Unity system provides voicemail to the main campus and larger remote sites." You can also network Unity Connection and Unity Express subscribers via VPIM. Make sure you confirm the versions are compatible.
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...