Insert a SIM card from a different carrier
Check if the phone connects to the new network
Make a test call, send a text, or use mobile data
Go to the phone’s settings and look for carrier or network lock status
On iPhone, check Settings > General > About > Carrier Lock
On Android, check Settings > Network & Internet > SIMs or Mobile Network
Contact the current carrier and ask if the phone is unlocked
Use the phone’s IMEI number to check with the carrier or manufacturer
Restart the phone after inserting a different SIM
Look for a message saying the SIM is not supported or invalid
Try activating the phone with another carrier’s SIM
Check the original purchase receipt or account details for unlock status