- I want to at least detect devices that are using 2G, UMTS or LTE.
- What would be the basic stuff, in terms of hardware and software, for that? I assume gnuradio or some Universal Software Radio Peripheral (USRP) would be a starting point for the software.
- But what hardware would you need to capture and emit signals?
- Could it at least detect the presence, if number or some form of ID of the phone are too hard to get.
- Could activity of cell-phones be classified (is it calling, getting data)?
Can you detect, or even join (as a device) a 2G, UMTS or LTE network using software defined cell-phones?
Modern base stations are software-defined. A very solid part of the processing happening in modern phones is software-defined.
I'd go as far as to say: If you've bought a phone in the last 10 years, it's arguable it's an SDR to some extent.
What would be the basic stuff for that? I assume gnuradio or some Universal Software Radio Peripheral (USRP) would be a starting point for the software.
USRPs are hardware, GNU Radio is software. You're comparing paper (as medium) and text (as what's written on the medium). (This analogy is a bad one, because the software doesn't run on the USRP, but anyway, your question reflects a lack of basic understanding of what which is.)
Bit what hardware would you need to capture and emit signals?
A USRP with appropriate frontend will do. Or a lot of other SDRs with appropriate front end.
How far could such a software defined cell-phone analysis go?
Far. (this question makes about as much sense as asking how far your book analysis with the ability to read can go.)
Could it take part as a cell-phone in the network?
Identical to your first question
Could it at least detect the presence, number, or some form of ID of the phone?
Identical to your first question. Also, this has nothing to do with SDR, but with the mobile communication standards. Obviously, a base station can recognize a phone.
Common standards (GSM, UMTS, LTE) do not associate the phone number with the hardware. That's done somewhere far away – the cellular network needs to know exactly nothing about phone numbers; it only needs to know which data to route to which device. Phone numbers are a couple of layers higher.
Could activity of cell-phones be classified (is it calling, getting data)?
Yes, trivially so: You can do that yourself by looking at a spectrum analyzer, in fact.
Using a package like Gnuradio and adding some sort of antenna for 2G, 3G or 4G, could a computer become a cell-phone?
Um, GNU Radio is software. Software doesn't plug into antennas.
2G, 3G and 4G are standards. Standards don't define antennas. Standards recommend some frequencies to operate on, and these define antennas.
You need to plug your antenna into some SDR device, and then run some software that implements the signal processing and call logic.
There's a few implementation of that for different parts in cellular stacks; I'm leaving the looking for that up to you, because this site is not for product/software/service recommendations.
At least, could it be a way of detecting a cell-phone nearby?
Or even detecting the number of this cell-phone?
No, because that info is not part of what goes over the air, or if it is, part of a much higher layer, which is usually encrypted.