They call a number of times but always hang up when transferred to answering machine

They have called 7 times so far to day all within 3h ours when the answering machine operates they will hang up It appears to be from a call centre

Call Centre of some sort. These guys change their calling ID so regulalrly it is difficult to block them. These days if I don't recognise the number I won't answer.

Message on answer machine .Background noise made it sound like a call centre and guy who called was singing.