If you don’t want to change SIM cards, you cold go to 2G and easily create a fake base station and disable 3G/4G on the phone. For 4G, it‘s a bit more tricky as you need to do a relay attack (https://alter-attack.net/) and even then only do some DNS redirection if you know what host is being looked up, or some fingerprinting based on the size of the traffic.
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).
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).