You have to wait for the network to broadcast the message, and then you will receive a +CBM unsolicited message. Note that not all networks broadcast all the time.
The modem will send (on the port you’re connected to) the message +CBM=xxxx when an unsolicited message arrives. It’s up to you to parse all the responses from the modem looking for the one you want.
You can’t control when (if) Unsolicited messages arrive. And these sorts of messages are very network dependent.
I suspect that if your network supports them (and you’ve subscribed to them) you have a good chance of getting one of these when your modem changes cells.