You'll need your iPhone's IMEI number if you want to unlock it and use it on a different cellular provider.
To verify this, find your iPhone’s serial number. Go to Settings > General > About, then long-press the "Serial Number" to copy it.
Chances are you haven't spent much time thinking about your Apple device's serial number. Whether you own the benchmark Apple Watch Series 9 or a budget-priced refurbished iPhone, your device's serial ...