Prefix 0400 506

Caller Type: Unknown

Got a call from this number shortly after I got a call from 0468990253 (Optus) who said nothing and hung up. The caller from this number did the same thing, said nothing and hung up. I called back and it went to an answering service with a female voice claiming the caller’s name is Heidi. I don’t know anyone called Heidi. Been getting lots of these weird calls lately. Obviously the “do not call’ register isn’t working. Seems suspicious.