In the notification center in iOS 7 calls that I've missed are staying in the notification center and can't be removed/cleared until I restart my phone.
Perhaps change/remove notification settings for Phone and then re-enable them? If it keeps on happening, perhaps reset all your settings (if not even reinstall)?
Ok I just restored in iTunes so it redownloaded the ipsw and then I set it up as a new phone, so there couldn't have been any settings carried over, and it didn't have the x there. restoring to my previous backup and submitting a bug report.