Rang this number back and got a recorded message, in an American female voice, saying the person at extension 5430 was unavailable and to leave a message.
I believe this to be a telemarketer who bought the service from Telstra and that Telstra sold this service (and hundreds more like it) in the full knowledge that this company was going to use this service to disguise who they really are.
As such Telstra is participating in unconscienable conduct in order to make money and is assisting scammers and other undesirable elements of our society to profit from the misery inflicted on our community by the low lifes who use these numbers to deceive and defraud thousands of Australians each year.
Number blocked.

Caller claimed to be from an Energy comparison company and claimed that I had inquired about electricity prices in the past. Had no details about my address, no details about my electricity account and wanted to sign me up. I do not remember making any inquiries about electricity prices and the entire call did not seem right. Caller did not offer a call back number or extension, just said that the number of call ID would work to call back, i have not tried and based on other comments regarding this number, I won't try.

got calls from (03) 7000 5428 and blocked it on my phone
now I get (03) 7000 5430 how high will they go?

I called back number (03) 7000 5430 reached voice mail but did not say for what company.

calls regularly but never leaves message