iPhone 6 won't make or receive calls and some texts after iOS 8.3 update?

OK, so I was having a few problems with my iPhone 6 not receiving all text messages and the messages app generally being slow and freezing, and getting stuck in "sending". So I updated my phone to iOS 8.3 in the hope it would fix the issue but it hasn't at all, and now I have the added problem of my phone no longer allows me to make and receive calls. When I try to make a call (either through contacts or dialling the number myself) and go to press the green call button, nothing happens! And when someone tries to ring me it doesn't ring on my end or give me a missed call. I have attempted to restore my phone 3 times now with iTunes, but iTunes insists on downloading the latest iOS first, but then fails after about an hour of trying to download. Anyone had any similar problems or have ideas for fixes?