What is the ruling on adding to the three greetings when seeking permission to enter?

Question:
Now, it is difficult for a believer to seek permission three times... one must do more, especially in houses where the doors are far away?

Answer:
One does the thing that conveys the message: such as ringing the bell, such as knocking on the ring, the thing that makes them hear along with the greeting.
Q: But... does one add more than three if one knows that they did not hear?
The Shaykh: As for the three greetings, that is the Sunnah. As for doing something else and adding, such as knocking on the ring, ringing the bell, or the like of that which alerts them; because they may not hear the voice.
Q: The bell?
The Shaykh: There is nothing wrong with it, because this is a matter of alerting, like the bell of a clock or the telephone; it is not intended with the pre-Islamic intention of hanging it.
Q: Does it prevent the angels?
The Shaykh: It does not prevent them, if Allah wills.