CME can be absolutely be integrated via H.323 with or without GK, SIP with or without register proxy, or even the two protocols concurrently.
Regarding the dialplan CME supports features like number expansions, secondary numbers, translation rules and the full set of IOS features that in short will let you define your dialplan as better you please.
Like Paolo said in his post, CME can be integrated to other CME/CM systems via a H323 gk or ICT trunk. A few things to note here is that Callmanager doesnt register phone DNs as e164 numbers. While CME by default registers all ephone-dn's as e164 numbers. So depending on the number of CME installations, you may want to choose either using only tech prefixes to do the routing or a combination of tech prefix/ e 164 numbers to do the routing.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...