It is a call centre. When you call back you are told all their operators are busy and they will call you back shortly. If you are calling back with a mobile they will capture your number and then keep calling you.

Keeps calling me and when I return call get a message saying all lines are busy.

Keeps ringing - leaves no message, can't be called back - you get 'hi - all of our operators are busy right now' and then it hangs up.

Called and seemed to accidentally trigger my voicemail. Sounded like a call centre in the background.