The history of this law dates back to 1935, when it was designed to protect post office staff who ran telephone switchboards from being harassed by the public, and it that context, it makes sense. Of course in 21st century the law doesn't make sense as it isn't humans transmitting our messages any more

Some legal commentary about its previous use: http://barristerblogger.com/2018/03/24/its-time-to-change-th...

While there is certainly a slippery slope argument about the validity of restrictions on speech, that is not how I see this - this is legislative debt, and an argument in favour of deleting deprecated laws