The departing Federal Communications Commission (FCC) chairman Kevin Martin announced another investigation is to be launched into Comcast shortly before leaving his post.
The regulatory agency is to focus on allegations that Comcast is deliberately downgrading its rivals’ phone services.
Among those whose sound quality is said to have been affected are Vonage and Skype, competitors of Comcast’s own VOIP service.
The FCC has ordered Comcast to reply to the inquiry by 30 January.
Comcast has responded by saying it has "fully complied" with the FCC’s so-called congestion-management practices and that it is "reviewing the FCC staff’s letter".
Julius Genachowski is expected to be named as the next head of the FCC.