T1, T2 and T3 are three terminals under the same SC node area and have all been successfully registered using the SIP protocol. When T2 and T3 are on a call, T1 calls T2’s number through the SIP protocol, then the SC will recover. Which of the following response codes should be given to T1?

T1, T2 and T3 are three terminals under the same SC node area and have all been successfully registered using the SIP protocol. When T2 and T3 are on a call, T1 calls T2’s number through the SIP protocol, then the SC will recover. Which of the following response codes should be given to T1?
A . 486
B . 483
C . 487
D . 408

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Answer: A