When a UC Administrator is troubleshooting DTMF negotiated by this SIP INVITE, which two messages are examined next to further troubleshoot the issue?

Refer to the exhibit.

When a UC Administrator is troubleshooting DTMF negotiated by this SIP INVITE, which two messages are examined next to further troubleshoot the issue? (Choose two.)
A . REGISTER
B . SUBSCRIBE
C . PRACK
D . NOTIFY
E . UPDATE

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Answer: B, D