I called back number (03) 8571 4086 spoke to a guy who said company name is Optus.
August 15, 2019
Caller: Optus NBN Just a reminder that i was getting upgraded to Optus NBN
July 5, 2019
Caller: Optus NBN person He was enquiring about switching my Optus cable internet to NBN cable with Optus. My paranoia made me verify with Optus that the number (03) 8571 4086 does actually belong to Optus Technical Support in Melbourne, Australia. It is an Optus Tech number.