Beware of the Telstra scam currently doing the rounds - I have had 3 calls in 3 days.
Call will appear to be from a genuine Aussie number (which it isn't) and the Indian caller will claim you have a problem with your Telstra internet modem
They then give you a license number of your modem and ask you to use the Windows cmd prompt, enter a cmd, and then point you to where the reference number is displayed. This is an attempt to convince you their call is genuine - reality is the licence/reference number is common to all Windows OS.
Now they think you believe the call is genuine they will ask you to go through several steps, which will effectively hand control of your PC to them.
Don't get sucked in.