Government will crack down on 45 companies, including telecom operators and a Chinese telecom equipment provider, which are yet to surrender their "off the air" monitoring system, an equipment used to tap phone calls, as it could have a bearing on national security.

In a recent meeting, the Department of Telecom (DoT) scrutinised the list of companies which had imported between 2007-10 and sent letters to companies which included some telecom operators to surrender as the possession of equipment was illegal, official sources said.

The government chalked out a list of 21 vendors which included 11 telecom operators, a public sector undertaking and a Chinese telecom firm, and served notices to them asking them to comply with the order of surrendering the equipment, the sources said.

CBDT provided Intelligence Bureau with a list of such equipment imported from four ports -- Nhava Sheva Customs, Chennai, Bangalore and Delhi and after this a list of 45 companies was drawn. Out of these, complete addresses were available in case of 21 companies whereas 25 others did not even have complete addresses.

According to sources, the Ministry of Home Affairs has been asked to crack down on some of the companies which have been ignoring repeated warnings of the government to surrender the equipment or provide reasons for its use.

"There is a need to investigate some cases and book them as per law which can serve deterrent to others," an internal noting said.

The government has already moved import of "off the air" in the "Restrictive List" from "Open General License" to make its entry into the country difficult, the sources said.