Thanks for using the Apple Support Communities. If you are having issues connecting your Bluetooth headphones to your new iPhone X, you may find the information and troubleshooting steps outlined in the following article helpful:
If you can't connect a Bluetooth accessory to your iPhone, iPad, or iPod touch - Apple Support
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Question: Help? My iPhone X isn’t recognizing my Bluetooth Skull Candy headphones...so I can’t event attempt to connect the devices.