The latest documents leaked by Edward Snowden, called “Spain last 30 days“, show that in a single month the NSA illegally spied on 60 million phone calls in Spain. Further details reported by the press mention that although calls were not recorded, location, dialled number, call duration and mobile phone serial numbers were all looked at by the NSA.
Based on that one can figure out that if the NSA was looking at mobile phones serial numbers they must have a way to link those numbers to people.
Mobile phone serial vs IMEI number
There are two kind of mobile phone serial numbers, IMEI (International Mobile Station Equipment Identity) and IMSI (International Mobile Subscriber Identity).
Mobile phone serial number and IMEI
IMEI numbers are embedded in the device, it will be displayed if you type *#06# on your dialpad, forming 15 or 16 digits, the software version called IMEISV contains 16 digits.
The first 8 digits of an IMEI number identify the model and phone’s origin, the remainder numbers are defined by the manufacturer and could be anything they want.
IMSI numbers contains 15 or less digits and they are embedded in the SIM card, the number is constantly send by your mobile phone to the network provider, enabling mobile phone companies to trace the phone using a technique known as triangulation. Tracing will work even if you don’t have GPS in your phone, triangulation relies on mobile phone network towers to locate you.
The first 3 IMSI digits contain the country code, followed by mobile network code and other numbers show subscription details.
For example if you go abroad the IMSI number will be used by the network to connect you to the foreign company that has a roaming agreement with your home network provider.
Both IMEI and IMSI numbers are transmitted to mobile phone companies. There are devices that can change a mobile phone IMEI number but in some countries like the United Kingdom this is illegal alleging that it hinders mobile phone theft investigations.
Stopping NSA metadata collection
With leaked documents showing that metadata is the main element used to flag calls by the NSA grid, using a calling card would should stop them from seeing the final numbers you are dialling, buying calling cards from a non USA company should add privacy.
It is probably rational to assume that the NSA knows about the calling card problem and receiving and making lots of calls using them with the same phone could raise a red flag in the system and mark you for further attention. Combining calling cards with different phone lines would be then a good idea if possible.
Another way that might fool NSA metadata collectors is by using a virtual phone number service like FlyNumber, where two people from Africa could communicate with each other using USA local phone numbers that are then forwarded to the phone of their choice or VoIP. Make sure it is not Skype, past documents showed that Skype is linked to the NSA PRISM global spying program.
As for stopping location tracking, opening your phone and taking the SIM card and battery off is the only secure way to do that. If this is too inconvenient then stick to VoIP calls tunnelled using a VPN.