The call made from a mobile phone was received at the Delhi police control room on Tuesday evening, they said, adding that the caller threatened to kill Dr Singh and Vajpayee.
During investigations, sources said, the number was traced to be active in Ghaziabad. However, they said, searches did not yield much result as the address used to procure the SIM card was found to be fake. The phone number, according to records, belonged to one Ram Avatar residing in Ghaziabad's Sanjay Nagar.
Raghubir Lal, superintendent of police (Ghaziabad), said a search was conducted in the area but could not find the address, as it was fake.