This is all I know about it. I got the information from a web site that was rather inaccurate. For example it said that letters were used only until 1948, but this was only the time when the new system was introduced, the changeover took until about the mid 1960ies.
When a friend of one of my younger brothers visited us (I guess he was 8 or 9 back then) suddenly our phone rang. The kid was completely confused until I answered the phone. He'd never heard a phone with a real bell in it.
