主代码写

package com.example.phonerecorder;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
    public void start(View v){
        //开启服务通过intent
    Intent intent = new Intent(this,Recordservice.class);   
    startService(intent);
    }
    //停止服务通过intent
public void stop(View v){
    Intent intent = new Intent(this,Recordservice.class);   
    stopService(intent);
    }
}

Recordservice类要继承Service成为一个服务类

由于是服务所以要在清单中设置

<service android:name=".Recordservice"></service>

package com.example.phonerecorder;

import android.app.Service;
import android.content.Context;
import android.content.Intent;
import android.os.IBinder;
import android.telephony.PhoneStateListener;
import android.telephony.TelephonyManager;
import android.util.Log;

public class Recordservice extends Service {

    @Override
    public IBinder onBind(Intent intent) {
        // TODO Auto-generated method stub
        return null;
    }

    @Override
    public void onCreate() {
        Log.e("tag", "开启");
        // TODO Auto-generated method stub
        super.onCreate();
        //创建一个电话管理器的对象(通过这个对象可以监听听电话的状态)
        TelephonyManager manager = (TelephonyManager) 
                getSystemService(Context.TELEPHONY_SERVICE);//传入一个电话服务
    //调用电话管理器的listen方法:注册监听
        //参数 : 1监听器   2要监听打电话的状态
        Listener listener = new Listener();
        manager.listen(listener, PhoneStateListener.LISTEN_CALL_STATE);
    }
    @Override
    public void onDestroy() {
        // TODO Auto-generated method stub
        super.onDestroy();
        Log.e("tag", "服务被销毁");
    }
    class Listener extends PhoneStateListener{
        //状态改变调用
        @Override
        public void onCallStateChanged(int state, String incomingNumber) {
            // TODO Auto-generated method stub
            super.onCallStateChanged(state, incomingNumber);
            switch (state) {
            
            case TelephonyManager.CALL_STATE_RINGING:
                Log.e("tag","响铃"+incomingNumber+"准备一个录音机");
                break;
            case TelephonyManager.CALL_STATE_OFFHOOK:
                Log.e("tag","接电话了"+incomingNumber+"开始录音");
                break;
            case TelephonyManager.CALL_STATE_IDLE:
                Log.e("tag","空闲状态"+"录音结束");
                break;
            default:
                break;
            }
            //录音需要用户的权限,在清单中添加READ_PHONE_STATE:读音状态
//          <uses-permission android:name="android.permission.READ_PHONE_STATE"></uses-permission>  
        }
    }
}

效果图

image.png