The reason why it doesn't matter about disconnecting the bell wire at that end, is because it'll be connected to nothing at that end anyway! So pulling a wire out of a terminal that doesn't connect to anything, doesn't change anything, it'll still be a disconnected end of a piece of wire. That's assuming you have a decent microfilter, which will make no connection to that pin in the phone socket. A bad microfilter might just pass the bell wire straight through to the telephone, completely unfiltered.