[Users] 180 Ringing / 183 Session Progress

Sam Lee sam.lee at super.net.sg
Tue Feb 6 11:08:37 CET 2007


Hey Guys,
 
Is it mandatory for SIP servers to generate to UA 180 Ringing to inform
them that the called party is now ringing ?
 
The reason i'm asking is because I am seeing strange results using
different UAs.
 
Using SIP clients to call out , instead of seeing 180 Ringing , i see
183 session progress even before call was picked up.
 
On some sip client , ring back can be heard.
On some others , ring back cannot be heard.
 
Is there any reason to suspect that these UAs needs to receive 180
Ringing before generating any ringback ?
 
Regards,
Sam
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