SIP protocol has two provisional messages types to indicate Ringing status which are 180 (Ringing) and 183 (Session Progress). The 183 Session Progress response indicates that information about the call state is present in the message body media information (SDP). The 180 Ringing message is a provisional or informational response used to indicate that the INVITE message has been received by the user agent and that alerting is taking place
Both 180 and 183 received messages may contain SDP which allow an early media session to be established prior to the call being answered. This early media session is usually used to hear Ring Back from remote device.
The new introduced feature provides control over 180 response in order to disable voice cut-through (generate local ringback from gateway/CUCM) even if SDP is contained in 180 response. This feature is “disable early-media”.
In CUCM this feature can be disabled under SIP profile.
In IOS Gateway this feature can be disabled using sip-ua.
sip-ua
disable-early-media 180
Now we understood the feature and understood the difference between 180 & 183 responses, but the question When 180 is generated and When 183 is generated?
The SIP gateway generates a 180 Ringing response when the called party has been located and is being alerted. For example CME that has SCCP/SIP phones at one leg and SIP Trunk on the other leg, it will generate 180 Ringing message on SIP Trunk after allocating the dialed phone and alerting it.
The SIP gateway generates a 183 Session progress response when it receives an ISDN Progress message IE with an appropriate media indication from PSTN. This progress IE is usually carried with ALERTING message (183) in SDP.
VERY IMPORTANT: CUCM can accept and handle 180 messages, BUT it will always generate 183 messages. CUCM can't generate 180 messages