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