Caller: I don't know

Called & got to my answering machine but didn't leave message. Any reputable business would leave a message wouldn't they? The day before I got 7 calls like this. I picked up 2 of them before the answering machine started& it was a call centre. First one said was NBN wanting to increase my internet speed. I hung up. Second one was call centre too. Said they were Telstra NBN wanting to boost my internet speed at no cost. I let them go on till they wanted me to access the "boost" from my pc. Hung up on me when I said no. All different phone numbers, but seem to be from same call centre (from noise in background).