iMessage Activation Unsuccessful

Hello.

I recently bought an iPhone 6 16GB and am running on iOS 9.3.1, and am currently having trouble with iMessage.

I have tried countless times now to activate iMessage, however the "Waiting for activation message remains for about 3 hours every time, after which it reads "Activation unsuccessful".

I also have another issue: when I open the "Send & Receive" box in the "Messages" settings folder, I see only my email address in the "You can be reached by iMessage at" list. There isn't even a "Start new conversations from" list on the page. What I want is to be able to enter my phone number into that.

I have tried everything I could find on Google and various forums. I will try and recall as many troubleshooting steps that failed as I can:

Turning iMessage off and on again

Rebooting the phone

Resetting network settings

Restoring the phone

I have made sure my number is stored in the "Settings > Phone > My Number" box. I have tried starting the number with both "0" and "+44" (I live in the UK).

My number is active as I have been able to call it and call from it.

I have "Send as SMS" enabled in "Settings > Messages".

Restrictions are turned off.

I am logged in to a valid Apple ID.

My Mobile Data is turned on and 4G is set to Data Only.

I should note that this is also the case with FaceTime, however I do not plan on using FaceTime at all so I'm not too fussed about that.

I could really do with some guidance because I don't know what I can do from this point on except take it in to the shop and ask them about it.

Thanks in advance,

Jamie