Hi, I am quite confused on how to categorize different protocols on different layers of OSI model. For eg. which layer do ICMP or DHCP protocols belong to? How to figure out on which layer do any protocol belongs to in OSI layer?

I have a topology like this- iPphoneA->cucm->switch(sw1)->router(R1)->router(R2)->switch(sw2)->IPphoneB. Both of my phones are registered to the Centralized CUCM.My question is: Do I need any trunk , gateway or dial peers on any router for the call t...