iPhone X unlock - how it works
Depending on where you bought your new iPhone X, it may be that the device is locked by a SIM lock. Nevertheless, in order to use the SIM card of another provider, it is necessary that you unlock your iPhone X.
Unlock iPhone X: How to check if the device has a SIM lock
If your iPhone X has a SIM lock, you can only use it with the SIM card of a specific provider. If you do not know whether your device has such a lock, the quickest way to check is to insert the SIM card of another provider into the device. If this is detected, your iPhone X does not have a SIM lock. If, on the other hand, an error message appears asking you to insert a valid card, the device is locked by a SIM lock.
How to unlock your iPhone X for use with another SIM card
If you have determined that your iPhone X has a SIM lock, but you still want to use it with another card or abroad, you have the option of unlocking the device.