6th phone call of the day re NBN now available and you are not connected so need to press to speak to technician or phone service will be disconnected in 24 hours. NBN has been connected for past 6 months. How can these phone calls be stopped. Past 2 days have been inundated with various numbers all with prefix "3"

5 x calls from numbers prefix 03 including the one reported above from approx. 9.30 am to 3.30 pm today Monday 5 August 2019 recorded message claiming to be from NBN and advising to press 1 to speak to technician otherwise my phone would be disconnected within 24 hours as NBN network is now available at my address and I have not yet been connected. (was connected 6 months ago )