About 250 lakh handsets are expected to be out of service from April 15, as GSM service providers, including Airtel and Vodafone pull the plug on them. These are unbranded Chinese mobiles that do not have IMEI (International Mobile Equipment Identity) numbers and pose a serious security risk.

All mobile phone service users have been directed by the Department of Telecom (DoT) to disconnect these phones. In fact, two deadlines - January 6 and March 31 - have already been missed by the companies. Now they have undertaken to acquire the necessary equipment to track these phones by April 15 and discontinue their services thereafter - a process that is expected to take another 15 days, that is, by April 30.

Here's why Chinese phones need to be banned.

Under law, all GSM phones are required to have a unique IMEI number that gets reflected at cell phone towers with which, if required, the location of a mobile phone user can be tracked. These Chinese phones, however, do not have International Mobile Equipment Identity (IMEI) number. In many cases, more than 1000 Chinese handsets have been known to have shared the same IMEI number.

Legal IMEI are 15-17 digits long and are unique in nature. This also means that no two cellphones should have the same IMEI. Chinese phones don't carry 16-digit IMEI numbers. At cell towers these phones show up as a string of zeros, or give out cloned IMEI numbers. That's why they pose a security risk.

The combination of IMEI numbers, or many phones with the same IMEI number, happens because the number is cloned in lots of 100, 1,000 or 5,000 phones by makers of unbranded Chinese mobile phones.

To check your IMEI no. of your phone, press *#06# on your mobile, the 15-17 digit IMEI no. will show on screen.

The phone service provider can tell your location by tracking the IMEI number from phone towers. It can tell where you went.

If you travel abroad, the foreign service provider knows that you are a visitor and from where you are originally Service providers have many mobile phone towers. During calls, the phone connects to the nearest towers. Even when not in use, the phone keeps contact with the nearest towers

By using signals from at least three of the nearest towers it's possible to locate a mobile phone accurately. The method, called triangulation, can be used by security agencies to track down any offender using mobile phone.

The security risk from unbranded Chinese mobiles can be guaged from the fact that a number of bombs have been triggered by terrorists by these phones. Mobile phones are part of terrorists' essential equipment, for getting instructions from their handlers or for passing on information. If they use legal phones, their location can be found by IMEI numbers.

To give an example, after the Mehrauli blast (in Delhi last year) the terrorists melted away without a trace. However, assuming that they had mobile phones, it should have been possible to track them down by zeroing in on all the phones that started to move away from the blast site immediately after the bomb went off. Instead of blindly putting roadblocks across the city, the security forces could have pinpointed all suspicious post-blast movements and caught the terrorists.

Security forces believe that, as it appears in the Mehrauli case, terrorists have taken to these unbranded Chinese phones to mask their movements. Currently, about 7-8 lakh Chinese phones come into the country every month. This figure was much higher before the talk of their ban started - in September 2008, 1.5 million of these phones came into India.