I eventually received a response from Optus, 91 hours and 27 minutes after I gave up on their telephone system and logged a complaint through their online system.
Only problem is that they want an email address that relates to their client. And as all of our email addresses go via our server and not theirs, well, this isn’t possible. At least, all of our email address relate to their client, but they don’t use the client’s name, so how are they going to know they relate. Perhaps there is a default email address that we were given with the ISP package, but I’ve got no records of it, which means I would probably need the assistance of their telephone help desk to find out what email address they had given us as a default. Catch 22.
And it gets worse. As you can see from the email I have copied below, if I can’t come up with an email address they want me to go to their telephone help desk anyway.

Dear Graham,
Thank you for contacting the Optus Business Centre.
Before actioning emails requests, we need to ensure our privacy guidelines are met. In this case, we require your email to come from an email address that reflects the business name on the Optus account. If you can please re-submit your email confirming this we will action and advise accordingly.
For further clarification, please contact the Optus Business Centre on
133343 or via email to inbusiness@optus.com.au.
Yours sincerely,
[Name suppressed so his kids won’t be teased at school]
SMB Wireline & Internet Support Services

I copied them the blog post, but obviously Optus is so large they don’t feat On Line Opinion.
Anyway, I wrote back:

Dear [Name Suppressed so his wife can still show her face at the school tuckshop]
Hazelhurst, the company on your invoice, does not have an email account.
I have spent over an hour on the telephone waiting for your call centre to respond, and I do not intend to go back to that.
You have the telephone number details for the phone at fault, and I can be contacted on 07 3252 1470. Your call centre can do whatever checks it wants to when someone rings me on that number.
Can I point out that I originally contacted you on Friday and it is now Tuesday? Your response times appear to be leisurely, to say the least.
I’d be interested in whatever deals you have to compensate customers for loss caused by your very slow response times.

If anyone really needs to ring us, they can Skype me! Which might be where we move next.