Called number back and was immediately answered. Australian accent male. He began saying "hello? Hello" while I remained silent.
There were at least 3 other people (male and female) in the background who I could hear were on calls to others.
Sounded like a small scale call centre, possibly out-of-home operation

No message left, went to Spam folder in phone

Never leaves a message

Hung up when I answered.

Called but left no message.

Call centre hung up when I asked who's calling