Check the phone’s original box or packaging
Look at the purchase receipt or invoice
Check your Google account device history for Android phones
Check your Apple ID account or Find My for iPhones
Check the carrier account or mobile service app
Check the SIM tray or back of the device if available
Check the battery compartment on older phones
Use the phone’s original warranty card or documentation
Check your email for order confirmation or shipping details
Contact the manufacturer with proof of purchase
Contact your mobile carrier for the IMEI on file
Check any device insurance records
Check backup files or device setup records if previously saved