On sites such as cell-track.com or phone-location.info, for example, it is easy to find out whether a device is abroad or not, or whether a device is currently switched on. All you need is the phone number. There's nothing you can do about it except keep your number secret. State actors also have other options, such as infecting the device with a zero-click exploit (state trojan). Only an anonymous SIM card and keeping your number secret will effectively protect you from government attacks.