Npower has been fined £60,000 for making abandoned calls to consumers, regulator Ofcom said.
The gas and electricity supplier made 1,756 abandoned calls and played messages containing marketing content over a seven-week period last year.
Ofcom said the harm caused by the calls was at "the lower end of seriousness". It understood that Npower would provide compensation to those affected.
The regulator said in a statement: "Taking into account this offer to compensate consumers, the number of occasions that Npower was non-compliant with the rules and the steps it has since taken to bring itself into compliance, Ofcom has imposed a fine of £60,000 on Npower."
Under Ofcom's rules, there is a limit on the number of abandoned calls organisations can make before it considers enforcement action.
An investigation found Npower exceeded an abandoned call rate of 3% of live calls over a 24-hour period on eight occasions between February 1 and March 21 last year.
Ofcom rules also prohibit organisations from playing recorded messages that include marketing content in the event of an abandoned call. It found Npower broke this rule on 1,906 calls.
Ofcom's consumer group director Claudio Pollack said: "Our rules are there to protect consumers from suffering annoyance, inconvenience or anxiety from abandoned calls.
"Organisations using call centres must comply with the rules or face the consequences. Where we find there to be breaches, even at the lower end of the scale, we can exercise our powers and take action."