我有一个VoIP应用程序，其中调用工作正常。
但是，当我们有一个活动的VoIP呼叫并且用户收到一个正常的PSTN呼叫时，活动的VoIP呼叫将变为空白-两边都听不到音频。
为了解决此问题，我们决定以编程方式结束新的传入呼叫，我成功地检测到了传入的PSTN呼叫。但当我试图结束通话时，我得到错误消息：

EndCallAction transaction request failed: The operation couldn’t be completed. (com.apple.CallKit.error.requesttransaction error 4.)

错误代码4是指CXErrorCodeRequestTransactionErrorUnknownCallUUID。
下面是我的代码：

// called during initialization
self.callObserver = [[CXCallObserver alloc] init];
[self.callObserver setDelegate:self queue:dispatch_get_main_queue()];

- (void)callObserver:(nonnull CXCallObserver *)callObserver callChanged:(nonnull CXCall *)call {
    // isCallInProgress static variable tracks if a VoIP call is active and I verified its value - works as expected
    if(isCallInProgress) {
        if(!call.isOutgoing && !call.hasConnected && !call.hasEnded) {
            NSLog(@"*** New incoming call detected while a call is in progress - so reject it");
            dispatch_async(dispatch_get_main_queue(), ^{
                CXEndCallAction *endCallAction = [[CXEndCallAction alloc] initWithCallUUID:call.UUID];
                CXTransaction *transaction = [[CXTransaction alloc] initWithAction:endCallAction];

                CXCallController *controller = [[CXCallController alloc] init];
                [controller requestTransaction:transaction completion:^(NSError *error) {
                    if (error) {
                        NSLog(@"*** EndCallAction transaction request failed: %@", [error localizedDescription]);
                    }
                    else {
                        NSLog(@"*** EndCallAction transaction request successful");
                    }
                }];
            });
        }
    }
}

接下来我可以尝试什么？