SOLUTION - this worked for me - I had tried everything mentioned here before.
I am in the UK, on O2, and had previously used the O2 TUGO App, however had long ago deleted it. It seems that TUGO still had my phone number registered, and they were somehow disrupting the Apple Activation Message sent to my phone (it was coming up as a regular SMS everytime I tried to activate rather than a system message.)
I reinstalled the Tugo App, Deleted my Account, Delete the App, Reset Network Settings - and then BINGO - iMessage and Facetime were able to instantly activate! PHEW.
Thanks a bunch O2 TUGO - you officially ruined the last 3 days of my life trying to fix this!