A class action lawsuit filed last week in U.S. District Court says Comcast violates antitrust law by refusing to provide digital voice-phone service to customers unless they rent a modem at additional cost.

Comcast’s digital voice customers, like the lead plaintiff in the case Robert Fromer, “have no choice but to pay an additional rental fee for a modem known as an ‘embedded media terminal adapter’ in order to utilize the DV services for which they pay a separate fee,” this lawsuit filed in New Haven says.

According to the lawsuit, even though Comcast purchases the modems from Motorola and Arris at a fixed cost it ties its digital voice service to the modem and refuses to sell the modem to its customers forcing them to rent.

The lawsuit filed by lawyers Anthony Medico and Frank Napolitano of Greenwich claims that Comcast has “sufficient economic and market power to coerce plaintiff and the Count 1 Class to pay rent for the eMTA modem and accept the tied products.”

They say because of Comcast’s position in the market “there is little incentive for other manufacturers to enter the market, and those that do are precluded from renting or selling their products.”

The lawsuit highlights Comcast’s “Frequently Asked Question” page where someone asks if they can purchase their own modem or if they have to lease it directly from Comcast.

According to the lawsuit Comcast responded by saying,” The only device that will work with your Comcast Digital Voice service is the Comcast supplied eMTA, which is delivered and installed by a Comcast technician. Currently Comcast Digital Voice is not compatible with equipment purchased elsewhere.”

A Comcast spokeswoman was unable to comment Tuesday on the claims in the lawsuit because it is pending litigation.