That message normally appears when a phone is reported stolen or lost. Are you saying that sometimes you can make and receive phone calls? Is your phone a Telstra supplied phone? Did you buy it new? and from Telstra? Have you tried doing a Network reset. ? Have you just purchased this phone and was it used by another person on anothet network? I think the first thing to fo is ring Telstra on 132200 or Live Chat and check the status of your phone with the IMEI number *#06#
A "Like" is always appreciated. If my advice is a solution, please mark it. I'm not a Telstra employee. 50 + years exp. in radio comms., eg mobile phones since 1963, two-way radios, base stations, antennas & pagers. My motto: "Doing the right thing is never wrong".