Call your mobile carrier immediately
Ask the carrier to suspend the SIM and block the device IMEI
Use Find My iPhone or Find My Device to lock the phone remotely
Mark the phone as lost in the device tracking service
Change your phone unlock PIN, password, and biometric settings if possible
Change passwords for email, banking, social media, and other important accounts
Sign out of the lost phone from all connected accounts
Remove the device from payment apps and digital wallets
Contact your bank if any financial apps were on the phone
File a police report if the phone was stolen
Notify your workplace if the phone had work email or company apps
Back up or restore your data to a new device
Monitor accounts for unusual activity