Question: In IMS, when is the list of codecs supported by the called UE available for reactive media transcoding invocation?
Option 1: During initial UE registration with the IMS network
Option 2: In the SIP response message after the session request has been sent to, and rejected by, the called UE
Option 3: When the Application Server invokes the MRFC for proactive media transcoding
Option 4: Upon completion of the media plane transcoding by the MRFP
Explanation: In reactive media transcoding, the decision to transcode is made after an initial failure (e.g., due to incompatible codecs). The called UE's supported codecs are communicated back to the network in the SIP response message, which is sent after the called UE rejects the session request due to codec mismatch. This information is crucial for triggering reactive media transcoding services.