Make sure you check out the live demo linked in the article.
The Motorola on the left is the attacker with the hacked firmware. The two phones to the right at your standard callers. With the firmware hack activated the Motorola can respond to the page request quicker than the intended recipient can, causing a denial-of-service.
The demo only looked for a certain caller to block, but potentially all callers from a single connected cell town could be blocked.
This is quite freaky. Not surprising, but freaky nontheless.
I do wonder though if the vurnerability is really new or that the companies already knew about this.