While there is real case that the UAC receives different SDP in 1XX and 200 OK,
and should not ignore the SDP in 200 OK.  Following is the case I met.

A, B, C are sip UE.
B owns sequential ring service(to C).
A calls to B, B rings but not to answer B,
timer expires, and C rings, C off hook.
A talking to C.

In the above case, when Invite request reach to B,
B may response with 183(SDP) firstly to connect A to a 
media server to hear announcement. While the first call
attempt will be cancelled, and when C off hook, A will
got a 200 OK (SDP) related to C.

thanks,
Nathan

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