Hi,
I was facing the same issue so I called apple customer care and they really helped in resolving the issue.
Please follow the steps below-
1) Turn your airplane mode on and off once.
2) Take your sim card out and insert it back while keeping the phone on.
3) Force restart your iPhone X (Push your volume up button for 1 second, then press your volume down button for 1 second and then press power/siri button until you see the apple logo, remove your finger as soon as you see the power button).
Note-My issue was resolved with this step.
4) Reset your network setting or reset all settings.(this will not delete the data but will reset few setting like wallpaper, ringtone and wifi setting).
PS-Apple is working on a fix and with next update they issue should be resolved but meanwhile resolve this.
Let me know if this works.