A major energy company and a home maintenance business have been accused by the communications watchdog of plaguing households with annoying cold calls.
Ofcom said npower and HomeServe used automatic calling systems to dial numbers, only to leave the line silent, hang up or leave marketing messages.
The regulator has told both companies it believes they "persistently misused an electronic communications network" after carrying out its own investigations. They have been given until August 10 to respond to the allegations "and to take steps to cease the misuse identified", or they could face fines of up to £2 million, Ofcom said.
Both companies allegedly made "an excessive number of abandoned calls" when they were investigated between February 1 and March 21 this year.
npower is also accused of leaving recorded marketing messages in the event of an abandoned call while HomeServe allegedly "made one or more repeat calls to specific numbers within 24 hours when a call had been identified by Answer Machine Detection equipment as being picked up by an answer machine".
They were served the notifications by Ofcom under sections 128 to 131 of the Communications Act 2003 after they were investigated for "causing annoyance to consumers through the making of silent or abandoned calls".
An npower spokesman said: "Our average abandon rate has been consistently below the regulatory 3%. Ofcom's concerns relate to individual non-consecutive days in which the rate exceeded this level. We believe that our prompt to make energy savings is fully consistent with energy policy and does not constitute marketing."
A spokeswoman for HomeServe said: "HomeServe confirms that this limit was exceeded by a single outsourced supplier. This fact was uncovered by our own internal audit processes, verified by independent, external auditors and reported to Ofcom on April 26.
"The problem resulted from the use of answering machine detection (AMD) technology, and was remedied immediately upon discovery. HomeServe takes the Ofcom regulations very seriously and as a result, AMD is no longer used on any HomeServe outbound calls, whether they originate in our own or in outsourced call centres."
Gillian Cooper, energy expert at Consumer Focus, said: "Unsolicited sales calls are a major concern for most of us. Few people will welcome being disturbed in their own home. Silent calls are even worse - not only are they annoying but they can cause anxiety, particularly for older and vulnerable people. Ofcom is right to threaten fines to any firm caught doing this. We don't agree with npower's defence - unsolicited calls, even if they include an energy efficiency message, are still marketing. npower and its subsidiary HomeServe must reconsider these sales tactics, given the negative impact they have on consumers."