My wife and I both have iPhone X on a Virgin Mobile contract. We are able to make calls to many numbers, but occasionally come across a number where we just get the message “You have dialled an incorrect number” when the number is definitely correct. One of the affected numbers is our son’s mobile, which is on the GifGaf network. Neither of our phones can make a call to his number. We have good (4G, full signal) reception. is this a known problem and what can we do about it?