在Android的CallLog中，subscription_id字段表示通话记录所属的SIM卡的ID。这个字段对于多SIM卡设备非常有用，因为它可以帮助我们区分不同SIM卡的通话记录。

下面是一个示例代码，演示如何获取CallLog中的subscription_id字段：

import android.Manifest;
import android.content.ContentResolver;
import android.content.pm.PackageManager;
import android.database.Cursor;
import android.provider.CallLog;
import android.support.v4.app.ActivityCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;

public class MainActivity extends AppCompatActivity {

    private static final int REQUEST_READ_CALL_LOG = 1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 检查权限
        if (ActivityCompat.checkSelfPermission(this, Manifest.permission.READ_CALL_LOG) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_CALL_LOG}, REQUEST_READ_CALL_LOG);
        } else {
            readCallLog();
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        if (requestCode == REQUEST_READ_CALL_LOG) {
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                readCallLog();
            } else {
                Log.e("MainActivity", "读取通话记录的权限被拒绝");
            }
        }
    }

    private void readCallLog() {
        ContentResolver contentResolver = getContentResolver();
        Cursor cursor = contentResolver.query(CallLog.Calls.CONTENT_URI, null, null, null, null);
        if (cursor != null) {
            while (cursor.moveToNext()) {
                String number = cursor.getString(cursor.getColumnIndex(CallLog.Calls.NUMBER));
                String subscriptionId = cursor.getString(cursor.getColumnIndex(CallLog.Calls.SUBSCRIPTION_ID));
                Log.d("MainActivity", "号码：" + number + "，SIM卡ID：" + subscriptionId);
            }
            cursor.close();
        }
    }
}

以上代码首先检查是否有读取通话记录的权限，如果没有则请求权限。然后在readCallLog()方法中，我们使用ContentResolver查询CallLog.Calls.CONTENT_URI获取通话记录的Cursor，并遍历Cursor来获取号码和subscription_id字段的值。