App 涉及到音频播放不可避免需要做系统来电处理。使用方法非常简单，但是不常用，有必要做个笔记方便使用时查看。
1.引入系统头文件

import <CoreTelephony/CTCallCenter.h>

import <CoreTelephony/CTCall.h>

2.声明来电监听对象
@property(nonatomic, strong) CTCallCenter* callCenter;
3.注册来电监听

- (void)initCallCenter {
  _callCenter = [[CTCallCenter alloc] init];
  _callCenter.callEventHandler = ^(CTCall* call) {
    dispatch_async(dispatch_get_main_queue(), ^{
      if ([call.callState isEqualToString:CTCallStateDisconnected]) {
        NSLog("CTCallCenter:Call has been disconnected");
      } else if ([call.callState isEqualToString:CTCallStateConnected]) {
        NSLog("CTCallCenter:Callhasjustbeen connected");
      } else if ([call.callState isEqualToString:CTCallStateIncoming]) {
        NSLog("CTCallCenter:Call is incoming");
      } else if ([call.callState isEqualToString:CTCallStateDialing]) {
        NSLog("CTCallCenter:Call is Dialing");
      } else {
        NSLog("CTCallCenter:Nothing is done");
      }
    });
  };
}