Consumers Union and Consumer Federation of America filed a letter on Friday with the Federal Communications Commission documenting actions by the two firms that in their view “appear to be contrary to preserving an open Internet.”

The groups claim Comcast has “imposed unnecessary obstacles” to prevent its customers from purchasing a cable modem made by Zoom Technologies. They also note a new 4G wireless broadband plan offered by MetroPCS could potentially discriminate against websites not affiliated with MetroPCS.

“The rules were adopted to help maintain an open Internet. As stakeholders prepare for the rules to take effect, the FCC should take a hard look at practices that may violate the spirit or letter of the rules in order to deter anti-consumer behavior,” said Parul Desai, policy counsel for Consumers Union.

The letter comes at a time when the FCC is facing a legal challenge to its net-neutrality rules from Verizon, which claims the commission lacks the legal authority to impose net neutrality.

The groups also ask the commission to check disclosures from both firms to make sure they conform to the new rules requiring broadband providers to disclose their terms and performance characteristics.