Android get Telephone Number

Question

I try to get the phone number with this function but it returns a empty string. Can anyone help me?

private String getPhoneNumber(){
        TelephonyManager mTelephonyManager;
        mTelephonyManager = (TelephonyManager) getSystemService(Context.TELEPHONY_SERVICE);
        return mTelephonyManager.getLine1Number();
    }

On create:

 protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_nuevo6);

                mP.setNumber(getPhoneNumber());
                Log.i("number",mp.getNumber());


            }
        });
    }