To investigate the SIP signaling to troubleshoot the problem, which field can provide a hint for troubleshooting?

End users at a new site report being unable to hear the remote party when calling or being called by users at headquarters. Calls to and from the PSTN work as expected.

To investigate the SIP signaling to troubleshoot the problem, which field can provide a hint for troubleshooting?
A . Contact: header of the 200 OK response
B . Allow: header if the 200 OK response
C . o= line of SDP content
D . c= line of SDP content

Answer: D