Why is Lifeline Renraku keeps door belling in my apartment?

Hi! I moved in my new apartment last year December and all my utilities or lifeline are already set up. I pay them monthly consistently and in time.

But recently there’s this people who repeatedly came and door belled while I’m at work and when no one answered by the bell, they transition to shouting “Lifeline Renraku”, what are these people? They always come weekdays which there’s no one here to answer because I’m at work. Also, they don’t leave any mail/notice for me to read later, hence I really have no idea about this. Has anyone have the same experience?