Unified CM and CME Connectivity

There are two primary ways to connect Cisco Unified CM systems and CME's between sites, that is H.323 or SIP. The primary focus in the SRND seems to be on the H.323 model which is deployed using either a Cisco Unified Border Element (CUBE) or what's called a via-zone gatekeeper. The important thing to note between CUCM and CME is that H.450 is used with CME for supplementary services but not with CUCM which can turn into dropped calls over a PSTN due to compatibility issues. I believe CUBE or a via-zone gatekeeper is used to mitigate these issues and act as the middle man. SRND is a pretty high-level document so a lot of things I read aren't going to really explain the reasons why or how. That's what's the CCNP: Voice books should provide for me...hopefully.