...they did suggest I change my number
"Telstra's best advice right there" - Said no-one ever.
I don't really have a need to openly and actively block any callers, and as Whoop pointed out, how is this (and the other call handling services Whoop mentioned) NOT a feature you can manipulate with your smartphone / via a web portal.
make a 2 second voice recording of silence. Make that their ringtone.
That's a good alternative for us 'dirty iPhone-types'.
I'd prefer the capacity to see and know when and how often they're calling, so something like this would work well if it were me.