Missed a phone call from this number, they left a long silent voice mail.

I called back number (07) 5648 0418 rang once then gets disconnected automatically.

Spam

Extremely poor line so unable to hear or understand anything even after switching on phone lout speaker.
In any case, I generally hang up on anonymous callers, so they would have to be very clear to get even a slight hearing from me.
If people call me anonymously then they can't be very interested in talking to me, so I NEVER call such callers.

Its a recording number. Since most people answer with yes, hello?

Missed a call from this number twice over 2 days. Tried calling bac and just received a message saying "All operators are currently busy, try calling back later"

Stayed silent on the line while I said hello a few times then just disconnected.