Cell-tower “SIM locators” generally only triangulate to within a few hundred meters—and if your phone’s off or the SIM’s swapped out, they simply can’t ping it.
On iPhone you’ve got Find My built in:
• Works even when the phone’s in power reserve or “offline” by leveraging millions of nearby Apple devices.
• Activation Lock prevents reactivation without your Apple ID.
• You can toss an AirTag in your case for extra precision.
Android has Google Find My Device too, but the ecosystem fragmentation and looser privacy controls can make it less reliable—and it won’t track a powered-off or SIM-less handset any better.