One of the reasons why Cisco CME uses the H.450.2 call transfer and H.450.3 call forwarding mechanisms is that it provides a standard means to support updating of the originating phone's display to track who the connected-to party is. Connected party display is often considered an important feature in business telephone networks. The connected party display is broader than the usual residential caller ID display you may be familiar with at home. Not only does it provide information about the calling party for inbound calls, but it also provides information about the called party for outbound calls. The connected party display can show you the name of the person you have called, for example. This is in contrast to simply displaying the number you dialed. Also, the standard caller ID display function provides a one-time indication of who the caller is only at the beginning of an incoming call. The connected party display feature allows for midcall updates of the connected party information that occur when the far end of the call performs a call transfer.
For example, after a caller has been transferred, his phone display can be updated to show the name and number of the extension he has been transferred to. Likewise, when you receive a call transferred to you by someone else, your phone display may initially show the caller ID of the person who is transferring the call to you. After the transfer is complete, your phone's display may update to show the caller ID of the transferee (caller).
When Cisco CME disables the H.450.2/3 mechanisms to interwork with Cisco CallManager, this display update mechanism is unavailable. However, Cisco CallManager has its own mechanism for performing connected party display updates. Cisco CallManager uses H.323 display and information IEs. Cisco CME also supports these IEs for performing connected party display updates.
The H.323 IE messages are carried as part of the H.323 signaling path. They are unaffected by the use of an MTP because they are informational messages only and don't generate changes to the call signaling state. This means that even when a Cisco CallManager performs an internal call transfer hidden behind an MTP, the H.323 information and display IEs are received by the Cisco CME system and are used to provide display updates. Not only can Cisco CME receive display IEs from Cisco CallManager, but it also sends display IEs to Cisco CallManager when Cisco CME performs internal transfer or forwarding.
Cisco CME sends these display IE messages for all H.323 calls regardless of whether a Cisco CallManager is involved in a call. This means that you can still get intersite connected-to party updates in Cisco CME networks where you have chosen to globally disable H.450 services.
Using H 450 x Tandem IP to IP Gateway