经过几个小时的调试，我解决了这个问题。我实现了一个CXCallObserver委托callChanged，在那里我检查另一个调用是否在我的tagoncall id的帮助下结束。如果另一个调用结束，我调用CXSetHeldCallAction来取消保留我的tagoncall，这样，音频会话就自动恢复了。

func callObserver(_ callObserver: CXCallObserver, callChanged call: CXCall) {
    guard let callId = voipCallId else { return }
    //check if another call got ended. if yes then unhold voip call
    if call.uuid != callId && call.hasEnded {
        let callController = CXCallController()
        let update = CXCallUpdate()
        let endCallAction = CXSetHeldCallAction(call: callId, onHold: false)
        callController.request(
            CXTransaction(action: endCallAction),
            completion: { error in
                if let error = error {
                    debugPrint("call unheld Error: \(error.localizedDescription)")
                } else {
                    debugPrint("call unheld Success")
                    //restore original video and mute status of call.
                    CallManager.shared.isShowVideo = self.originalIsShowVideoStatus
                    CallManager.shared.isMuteAudio = self.originalIsMuteAudioStatus
                    self.provider?.reportCall(with: callId, updated: update)
                }
            })
    }
}

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