Why were bells hung on tombstones in early foreign countries?

What was the purpose of hanging bells on tombstones outside crosses in early foreign countries? It can be seen in American TV series that the cross tombstone in early foreign countries will have a bell attached to one side (for example, there is a very obvious picture in the recent Westworld, the period is probably shortly after the Civil War, but it seems that there is no longer any now) and I want to know why. Baidu had passed before, but do not have the answer, estimate this problem needs the talent that lives abroad for a long time to know likely.

These tomb or graves are called safety coffins because medical science was not very advanced in the past. Sometimes a patient would fake his or her death, but people thought he or she was really dead, so they buried him or her. In addition, cholera was prevalent at that time. When the man woke up in the grave, he was buried, and through the thick soil and the top of the coffin, no one outside could hear him. So he was buried alive in the grave.

When people realized this (because of a famous philosopher John Duns Scotus A bell is attached to the front or side of the gravestone, and a rope (or other mechanical device) is attached to the inside of the coffin. If the person is mistakenly buried alive because of false death, they can pull the rope inside the coffin and ring the bell for help. Some SafetyCoffin use flags or fireworks instead of bells.