Cisco Call Manager

For call set up, teardown and supplementary service work, CUCM uses SIP or SCCP to communicate with Cisco IP Phones. Use the Real-Time Transport Protocol (RTP) to deliver audio, after a call has been made, media exchange occurs directly between the Cisco IP Phones across the IP network. After the call has been set up, CUCM is not involved in a call. If during the call period the CUCM server were unplugged, users would never notice it unless they were trying to use a feature on the phone. CUCM is only involved in the setting up, teardown and functionality of the call. If the CUCM server that installed the call was down, end users would see a message on the LCD screen of the IP phone that read “CM Down, Features Disabled.”

Read more: how does cisco call manager work

At the start of a call, an IP phone user A collects the handset and sends a message to CUCM, which tells CUCM it has gone off the hook. CUCM answers this stimulus by answering a message telling that the device plays a dial-tone file stored in the phone’s flash memory. The user listens to the dial tone on phone A and begins to dial phone B. SCCP phones send their numbers to CUCM when they are pressed, while SIP phones send dialed numbers by default into a single message (signaling). SIP phones have options for using SCCP (keypad mark-up language[ KPML] and dial rules) similar to SCCP phones. Digital analysis of the dialed digits is carried out by CUCM. When a match is detected, CUCM routes the call according to its setup. If a match is not found in CUCM, the caller will receive a reorder tone.

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