Brian Gaff formulated on Sunday :
Yes the transformers are just that. as I say its simply operating a solenoid
and has switches to stop it or springs to make it go both ways. The reason
a ding or a dong is not working is usually switch, spring or dirt related.

There are no switches, apart from the bell push contacts. The solenoid
slug is pulled one way to create the ding, as the button is released
the slug is pulled by the spring against the 'dong' bar.