In an electric bell, we generally use an electromagnet. The reason is that we only require that magnet to exhibit magnetic property only when we wish to ring the bell. However, a permanent magnet does not exhibit such behavior. Once it starts showing magnetic property, it shows the property for quite a long time. After some time it loses its magnetic property, which cannot be regained.

Therefore, a bell ringing would ring for a long period and would only stop when the permanent magnet loses its magnetism, but it cannot be turned off by pressing the bell.