VoLTE网络中，ISUP 消息 REL 消息中那几个原因值对应于SIP 错误响应值480

VoLTE网络中，ISUP 消息 REL 消息中那几个原因值对应于SIP 错误响应值480

A.17 user busy

B.18 no user responding

C.19 no answer from the user

D.20 subscriber absent

正确答案是BCD