1) Does the request sent to CUCM first?
2)What cucm will do?
It depends on configuration. In most simple case, there is no CUCM at all, thus it do nothing.
3)When DNS get involves in this.
4)What DNS will do
It depend on configuration. In most simple case, IP address is dialed, DNS is not used at all thus it do nothing.
These questions may be simple to you
It's no longer simple question, it's generic question. Thus it can't be answered by simple example.
Just "SIP VC endpoint call flow" is simple question. Originator sends SIP invite with sdp directly to called party identified by IP dialed by user. Neither CUCM nor DNS is used. Called phone responds with SDP. Originator sends ACK, call is considered connected, audio and video data are translated according SDP. Well, even this question can't be answered with no knowledge of configuration, as content of both SDP depends on configuration.
Complex scenario with CUCM is not simple question. And I'm no expert on it as our phone network uses no CUCM.
Consider to disclose purpose of your question. It may help an expert to answer.
