finding my cell phone signal in the spectrum

[John Byrne]

Hi, have just started learning about gnuradio and sdr in general. I have a hackrf one and I've been using it to experiment. I was hoping I'd be able to locate my cell phone's signal, just out of curiosity. I'm not looking to do anything with it - just see it. But it's proving to be harder to find than I thought. Could anyone offer any tips to help me find things like this?

I hoped it would be like this video - around the 4:00 mark he starts a call and you can see a clear signal of around 5Mhz wide:

https://www.youtube.com/watch?v=aFHWAEb3sn4&t=268s

But I can't find anything similar on the gnuradio companion FFT or waterfall displays that corresponds to making a call on my phone.

Regarding which band to look at, I'm in the US and using T-Mobile. I've looked at their info here (https://www.t-mobile.com/support/coverage/t-mobile-network) to get some ideas of where to look. This is where it gets tricky for me. How do I know which service my phone is using? I usually have 4G mobile data. But my phone was bought as an "international" model and according to T-Mobile, based on the IMEI it doesn't support VoLTE. So that leaves 3G and 2G for calls. I know from experience that I don't get mobile data when I'm on a call, so based on the above web page, that only leaves 2G, which they say should be 1900Mhz. So that's where I've done most of my searching.

I wonder if the problem is that the channel is so narrow that I don't see it. In the video above, the guy gets a ~5Mhz wide signal, but my 2G call is presumably much narrower than that right?

Another thing I tried was running a program called kalibrate-hackrf that scans for base stations and shows you the frequencies. It does find lots of them in the 1900Mhz band, but I seem to get different results every time I run it. And I still couldn't find my signal.

I just want to see something that turns on/off when I start/end a call. Initially I wanted to do it out of curiosity - now I'm determined to do it because it's proving so tricky. Is it reasonably to expect to find this? Or is there some kind of frequency hopping that makes it impossible to see? Any suggestions for how to go about it? Thanks!

-John