Caller: I don't know

I've received about 6 calls over the last 3-5 days for different numbers, which I usually let them ring out. However I decided to answer this one and say nothing. Low and behold, 3 seconds later, the line was disconnected by them.
Similar numbers which I have received, with no call back txt message.
(02) 8290 3611
(02) 8087 9000
(02) 8457 6412
(02) 8086 0482
(02) 7735 0193
(02) 8869 2942