I wonder how practical it would be to add something like a limeSDR/USRP/HackRF to a tinycheck to be able to at least detect this. I doubt it would be affordable for most individuals but I could see cost justification for orgs.
Of course, you can also just check if the phone sends something by looking at the RF energy or even build an uplink decoder, but I doubt that this is very useful information by itself for this use case.
Finally, what I propose instead, is to use a private LTE network, which you can create using a SDR and srsLTE and some programmable SIM cards, which you need to insert into the phone. This way, it‘s easily possible to view any traffic leaving the phone on any connection. Plus, srsLTE has been shown to work on Raspberry Pi as well (I think).