Caller: No idea but underneath the number it had "Sydney"

This caller didn't answer my call. Since the beginning of January 2020 I've had 18 similar calls, all with different numbers, some +61, with various places listed under their numbers, eg. Sydney, Parramatta, Westmead - when I answer there is dead silence, no engaged tone so I'm puzzled why they bother to call.
I usually call them back and get the message, this number is not connected, please check the number and dial again, or similar.