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.