Rang them back. Got their voicemail, Indian accent - so likely a call-centre. Their identity deliberately not revealed by noise: "Thx for calling (fuzzy sound) ... we are not available now, please leave your ...."

Most likely a Telemarketer or SPAM call.

Claiming to be a company called energy savers - wanting to give customers a new free hot water service

rang briefly, left no message

probably scam caller