And with the iPhone X the process is different because there is no Home button.
With iPhone 7 and 8 the process is different because the Home button is software. With older iPhone models you could hold down the Home and the Sleep/Wake buttons together for about 10 seconds. The way you reboot or reset your iPhone depends on the model you have. It is possible that the SIM is not sitting in the SIM card tray correctly, so reinsert it and see if that makes a difference. Check the status of Apple’s activation server by visiting Apple’s System Status page here. It is possible that the servers at Apple are busy (especially if there is a new iPhone or an update to iOS). You can’t activate an iPhone without a SIM card. How to iPhone Fix Activation Errors Check that there is a SIM card in your iPhone