如何监听电话状态

[plain] view plaincopy#import <CoreTelephony/CTCallCenter.h>  #import <CoreTelephony/CTCall.h>  [plain] view plaincopy-(void)detectCall  {       CTCallCenter *callCenter = [[CTCallCenter alloc] init];      callCenter.callEventHandler=^(CTCall* call)      {          if (call.callState == CTCallStateDisconnected)          {              NSLog(@"Call has been disconnected");              self.viewController.signalStatus=YES;          }          else if (call.callState == CTCallStateConnected)          {              NSLog(@"Call has just been connected");          }                    else if(call.callState == CTCallStateIncoming)          {              NSLog(@"Call is incoming");              //self.viewController.signalStatus=NO;          }                    else if (call.callState ==CTCallStateDialing)          {              NSLog(@"call is dialing");          }          else          {              NSLog(@"Nothing is done");          }      };  }