Check the original phone box
Look at the SIM tray or back panel if printed
Dial *#06# on the phone
Open phone settings and find About Phone or General > About
Check the purchase receipt or invoice
Look in your Google account device list or Apple ID device list
Check the carrier account or mobile service provider records
Use the Find My iPhone or Find My Device service if the phone is linked
Contact the network provider to report the IMEI
File a police report and provide the IMEI
Use the IMEI with the manufacturer’s support if the device is lost or stolen