A Technical University of Berlin telecommunications security research group successfully blocked local cell phones from receiving calls and text messages by making software modification on a GSM phone.

Using Motorola phones to test the activity, researchers could block calls and text messages intended for people sharing a common network within a location.

Jean-Pieree Seifert, the project lead said that the software modification can work on 2G GSM networks that is the most common type of cell network used worldwide. In the U.S., both AT&T and T-Mobile use GSM networks.

Seifert heads a telecommunications security research group at the Technical University of Berlin. Seifert and team presented a paper on the technique at the Usenix Security Symposium in Washington, D.C., last week.

Seifert said that just a phone can block services to all subscribers provided by base stations within a location area.

By modifying the phone's embedded software or "firmware," on a chip called the baseband processor, researchers were able to prevent the network from delivering incoming calls or SMS messages to the intended customer.

According to Seifert, normally, when a call or SMS is sent, a cellular tower 'pages' nearby devices to locate the right recipient and usually the right phone answers that particular page, saying 'it's me.' Then the call or SMS is allowed to pass through.

However, the modified firmware blocks calls because it replies to the network pages quicker, by saying 'it's me' than the recipient's phone. As a result, the right recipient never actually receives the message or call.

"If you respond faster to the network, the network tries to establish a service with you as an attacker," said Nico Golde, a researcher in Seifert's group. That's enough to stall communications in a location area."

Researchers claim that 11 of these modified phones can be used to block services to a location area of E-Plus, the third-largest network in Germany. GSM network protocols would require modifications such as use of encrypted codes to stop such deceptions and to allow intended recipients to receive information.