Radio professionals - is it possible to know cell phone ID's of users within 100 meters using a device?

Question

I heard modern cell phones transmission pulses to the towers are encrypted and are only temporary ID's on a forum called halfbakery. Is that true? If I had a device that wanted to know the cell phone's permanent ID's, would I need the cellular company's cooperation with the devices?
Thanks

Answer 1

It is true that cell phones broadcast a walsh code, so the messae going out isn't so much encrypted, as much as it is encoded. Yes, the base station assignes a frequency and a code for your call.
You can buy a device that communicates with cell phones, little network bridges and such. Some offices have these for short range radio and walkie talkies, that comes with base station code that monitors band usage. If you want to walk into a city and start reading nearby cell phones though, you are going to make the phone companies very unhappy if you get caught. I dont know what the jerks would want from you in that case, I doubt they would give you permission unless you are some large government agency.

Answer 2

there is a device simulating a station on short range, causing the cell phone to re-login to the real station.
In this process the phone airs specific data which (at least for GSM-phones i know) can be found in the specs.
i'm sorry for not knowing wether that transmission includes just the phones IMEI and/or number, or wether its encrypted at this basic level or not.
i'm very sure it can be found in the data-sheets being available on the net somewhere.