There are two numbers on the cellular network that matter: international mobile equipment identifier (IMEI). This identifies the cellular radio in question, usually the phone but a dual SIM phone will have one for each slot.
The second is the international mobile subscriber identifier, IMSI. This is the identifier the SIM sends to ask the network for functionality.
Even without a sim installed, the phone may transmit and will transmit its IMEI when doing so. This is so that cell towers can talk back to the device (a bit like ssids in WiFi networks). As mentioned in other replies to you you can often dial emergency numbers and your calls are routed. To do that you need to know which device is calling.
So yes you can track individual phones. You can also tell when a phone has changed SIM or a SIM has changed phone and so on. No idea if networks do this, but the data is there.
The second is the international mobile subscriber identifier, IMSI. This is the identifier the SIM sends to ask the network for functionality.
Even without a sim installed, the phone may transmit and will transmit its IMEI when doing so. This is so that cell towers can talk back to the device (a bit like ssids in WiFi networks). As mentioned in other replies to you you can often dial emergency numbers and your calls are routed. To do that you need to know which device is calling.
So yes you can track individual phones. You can also tell when a phone has changed SIM or a SIM has changed phone and so on. No idea if networks do this, but the data is there.