Services like Apple’s "Find My" or Google’s "Find My Device" are the most effective free ways to track a phone. However, users at Apple Support Community note these rely on account login rather than just the IMEI.

Log into your iCloud or Google account immediately to secure your data.

Ask them to "blacklist" the IMEI so the thief cannot sell the phone as a working device.

IMEI Tracking Software Used by Police: Is There a Free Version?