When you telephone somebody an initiation protocol occurs. One of these is SIP. The older ones had grown over time and SIP was an attempt to create a common protocol. The best way to think about SIP is like email. Emails use a domain address me.com and all it is fine until I want to email you.com. In the real world we normally retain extensions for phones so email@example.com wants to telephone firstname.lastname@example.org. Me could e a CME and You could be a CUCM.
SIP trunks have two meaning they can like CME to CME or CUCM. The other meaning is a SIP to PSTN gateway. Cisco routers in corporate environments but specialist equipment in providers.
Finally because the SIP trunk comes into your enterprise they often have special firewalls called session border controllers.
Traditionally phone manufacturers had their own protocol from phone to phone system but these have generally moved to SIP but H323 is still used.
So, in this Case we can not Call from email@example.com to h.323 or vise versa ?
1. how the Telecome Providers Connect h.323 to SIP Network ?
2. when we uses Private Cell Phones which protocol dose it uses to Connect Telcommunication ISP
3. is it Possible to uses CUCM for Cell Phone Registration and Call routing ?