I am wondering whether IP phones are aware of the directory numbers assigned to them or whether only the call routing component residing on the call manager knows the directory numbers assigned to the attached IP phones.
If IP phones knew their assigned directory numbers and transmitted them between themselves via the signalling channel, it would be impossible to set up dialing transformations. Is this point of view correct or don't IP phones have direct contact between themselves via the signal channel? (I think I learned that they should have direct contact in the last phase of an call establishment).
The IP phone itself acts as an endpoint that registers with the call agent (CUCM or CME for example). The callmanager will keep a record of the IP phones MAC address and the associated DN so calls can be routed between phones. Basically when you place a call between Phone A and Phone B, messages are exchanged between the IP phone A and CCM, the CCM routes the call to IP phone B and at the end the RTP media channel is established between the 2 phones/endpoints.
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