It’s no trick to reveal the “unique number”. It’s analogous to a PC MAC address - out in the open.
It isn’t just the URD - read the article. They claim that capturing the URD is part of the program, but not all of it. The device can disrupt calls, but they (authorities per the article) made software changes so that 911 calls don’t get disrupted.
Another article I found claims that:
The program is more sophisticated than anything previously understood about government use of such technology...
Newer versions of the technology can be programmed to do more than suck in data: They can also jam signals and retrieve data from a target phone such as texts or photos. It isn’t clear if this domestic program has ever used those features.
Source: http://nymag.com/daily/intelligencer/2014/11/new-spy-program-planes-cell-data.html
I don’t think MAC addresses transmit.