America's top telecommunications companies are refusing to say whether they accept that the bulk collection of their customers' phone records by the National Security Agency is lawful.

The phone companies are continuing to guard their silence over the controversial gathering of metadata by the NSA, despite the increasingly open approach by those at the center of the bulk surveillance programme. On Tuesday the secretive foreign intelligence surveillance (Fisa) court declassified its legal reasoning for approving the NSA telephone metadata program periodically over the past six years.

Verizon, the telecoms giant that was revealed in June to be under a secret Fisa court order to hand over details of the phone records of millions of its US customers, was one of the firms that declined to answer Guardian questions relating to the legality of the scheme. AT&T, Sprint and T-Mobile US also declined to comment.

CenturyLink, a multinational company based in Monroe, Louisiana said: "At CenturyLink, we respect and protect the privacy of our customers and only provide information to the government when required or permitted by law. We do not comment on matters of national security or specific government requests for information."

In its declassified opinion, the Fisa court revealed that no telecoms company has ever challenged the court's order for the bulk collection of phone records. The opinion, written by Judge Claire V Eagan, implied that by failing to challenge the legality of the programme, the phone companies were passively accepting it its constitutional status.

Seeking clarification, the Guardian asked five of the top US telecoms firms whether their lack of resistance to the collection of their phone records was indeed an implicit acceptance of its legality.

The Guardian also asked how the phone companies could justify to their own customers the decision not to challenge the court orders, in stark contrast to some internet companies such as Yahoo, which have contested the legality of NSA collection of their customers' data.

The phone companies were asked by the Guardian to make clear whether they felt their compliance with Fisa court orders relating to NSA data collection was voluntary, or whether they felt pressured by any party into conceding without legal protest.

The companies' decision not to comment on any aspect of the NSA dragnet puts them in a increasingly peculiar position. By withholding their internal views from the public, they are setting themselves apart from equivalent internet firms that are taking a more bullish stance, and are shrouding themselves in more secrecy than even the Fisa court, one of the most tight-lipped institutions in the country.