Indian female voice said that she was from Telstra and she was calling about my internet connection. I told her that I didn't have any internet and she hung up very quickly. Almost on a daily basis we get "Private Caller" calls, but this was the first from this number.

I called back 'your call could not be connected please check the number and try again'

your internet is to be cancelled

Female caller with Foreign (possible Asian) called to tell me that my internet was going to be disconnected (for the 6th time in the last 3 months). I try different tactics each time, this time I told her I wasn't with Telstra but Optus, this was not true, but she would have known if it was legit but instead she hung up immediately, like they always do.
Does that sound like a professional company like Telstra?
What the scammers are doing now they are transmitting false phone numbers to register as an incoming call ID and getting around the "Private Caller" blocking feature. But what is more worrying is that they sometimes use genuine numbers, causing considerable grief to the genuine owner. It is possible that they are "paying back those victims" that give them a hard time by using their phone number.