Android应用中怎么实现一个通话录音功能

package com.eboy.phoneListener;import java.io.File;import android.app.Service;import android.content.Context;import android.content.Intent;import android.media.MediaRecorder;import android.os.Environment;import android.os.IBinder;import android.os.Vibrator;import android.telephony.PhoneStateListener;import android.telephony.TelephonyManager;import android.widget.Toast;public class phoneService extends Service {  @Override  public IBinder onBind(Intent arg0) {    // TODO Auto-generated method stub    return null;  }  @Override  public void onCreate() {    // TODO Auto-generated method stub    super.onCreate();    TelephonyManager telephonyManager = (TelephonyManager) getSystemService(Context.TELEPHONY_SERVICE);    telephonyManager.listen(new PhoneListener(), PhoneStateListener.LISTEN_CALL_STATE);  }  private final class PhoneListener extends PhoneStateListener{    private MediaRecorder mediaRecorder;    private File file;    private Vibrator vibrator = (Vibrator) getSystemService(VIBRATOR_SERVICE);    @Override    public void onCallStateChanged(int state, String incomingNumber) {      try {        switch (state){        case TelephonyManager.CALL_STATE_RINGING: //来电          break;        case TelephonyManager.CALL_STATE_OFFHOOK: //接通电话          file = new File(Environment.getExternalStorageDirectory(), incomingNumber + System.currentTimeMillis() + ".3gp");          mediaRecorder = new MediaRecorder();          mediaRecorder.setAudioSource(MediaRecorder.AudioSource.MIC);          mediaRecorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);          mediaRecorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);          mediaRecorder.setOutputFile(file.getAbsolutePath());          mediaRecorder.prepare();          mediaRecorder.start();          Toast.makeText(getApplicationContext(), "电话已接通，开始录音。", Toast.LENGTH_SHORT).show();          //震动一下          vibrator.vibrate(100);          break;        case TelephonyManager.CALL_STATE_IDLE: //挂断电话          if (mediaRecorder != null){            mediaRecorder.stop();            mediaRecorder.release();            mediaRecorder = null;            Toast.makeText(getApplicationContext(), "电话已挂断，录音停止。", Toast.LENGTH_SHORT).show();            //震动一下            vibrator.vibrate(100);          }          break;        }      }catch (Exception e) {        e.printStackTrace();      }    }  }}