Calcutta, Aug. 5 :    Calcutta, Aug. 5:  Cops on the 'cellphone trail' of Parthapratim Roy Burman's abductors have drawn a blank. It has now been established that the abductors had been keeping in touch with family members of the Khadim's vice-chairman over their cellphones - all fitted with 'international roaming pre-paid cards from foreign countries' - to foil any attempt to trace the calls and track their movement in or around Calcutta. According to information available with the crime branch of Mumbai Police and the CID, the gang was using pre-paid SIM cards from Dubai, Dhaka and London. Sources said the kidnappers were aware that the police would not be able to trace the incoming calls as the numbers would not flash on the caller-line identification (CLI) machines installed in either the local cellphones or the landlines in Calcutta. Telecom department officials confirmed that calls from international roaming SIM cards of foreign countries were made from Calcutta and the suburbs to some cellphones here and to the Roy Burman residence. They informed the police that 'calls made from SIM cards of foreign countries cannot be traced as there is no international agreement between India and the rest of the world'. According to R.K. Mishra, a senior officer of Calcutta Telephones, CLI facilities on international roaming cellular cards are available 'only if there is an agreement between the countries involved'. VSNL authorities have also told the sleuths that they can do little to 'trace the numbers'. As a VSNL official put it: 'The city is divided into several cellular zones. When a call is made from a cellphone, it is recorded in a master computer. But in this case, calls will be recorded without the numbers.'' Officers of the Mumbai crime branch and the CID said the kidnappers were using four foreign SIM cards. 'We have informed Calcutta Police that Roy Burman's kidnappers were using two SIM cards from Dubai and one each from Dhaka and London to interact with the victim's family and among themselves,'' a crime branch officer of Mumbai Police said over telephone on Sunday. Investigations also revealed that the kidnappers had accomplices in Hyderabad, Lucknow and Patna to whom frequent calls were being made. VSNL sources said 'around 120 calls' were made from the two Dubai SIM cards. Each call would have cost the kidnappers around Rs 80 per minute. Attempts are on to ascertain the number of calls made from the Dhaka and London SIM cards. 'The entire operation was carried out with the help of these cellphones. Just keeping in touch with their associates and the Roy Burman family over the eight days must have cost the abductors around Rs 1 lakh,' a CID officer said.