It is investigating the firms, which have over 9 million customers between them, for subjecting members of the public to too many silent or abandoned marketing calls.
Ofcom says it believes they "persistently misused an electronic communications network" by making an excessive number of abandoned calls.
Call centres use automatic diallers to contact large numbers of people. But the person at home can find there's no one on the line if there aren't enough staff at the call centre.
It added that Homeserve repeatedly called people within a 24 hour period and that Npower played recorded marketing messages if no one from the call centre came on the line.
Ofcom can impose fines of up to £2m. It is considering whether to take further action.
Homeserve admitted to the BBC that it had been in breach of the regulations for a very short period, but a spokeswoman said the company was now fully compliant.