It's British Gas.
Probably originates from the same Asia-based call centre from which they ring to hassle you into making overdue payments.
Seems like a very cynical ploy to get you to cough up - essentially bullying through nusiance calling.
This call only happened when I had a bill with them overdue, and followed a call from a call centre demanding payment.
Pay your British Gas bill, and I guarantee you, the calls will stop.