In normal situations, when a call or SMS is sent over the network, a cellular tower “pages” nearby devices to find the one that should receive it. Normally, only the proper phone will answer—by, in effect, saying “It’s me,” as Seifert puts it. Then the actual call or SMS goes through.

The rewritten firmware can block calls because it can respond to paging faster than a victim’s phone can. When the network sends out a page, the modified phone says “It’s me” first, and the victim’s phone never receives it.

“If you respond faster to the network, the network tries to establish a service with you as an attacker,”

via Software Update to $20 Phones Could Topple 2G Cell Networks | MIT Technology Review.