Q: I have recently bought an iphone 6s and I am having a hard time activating my number with imessage/facetime.

What country are you in and who is your carrier? iMessage and FaceTime activation use SMS messages to Apple's activation servers in the UK. If you are using an unsupported carrier, it may not work. If you are using a supported carrier, make sure they have your account provisioned properly for an iPhone.

If you get an error when trying to activate iMessage or FaceTime - Apple Support

Follow this article to the letter and remember that when allowing to the iPhone to have 24 hours to attempt to activate, it cannot be toggled off and on again, or you're refreshing your activation queue. If after following the steps explained does not allow for your iMessage and Facetime to activate, Contact Apple for support and service - Apple Support. So long as your carrier presently has no blocks on short codes or premium texts and is setup for this service, there should be no problem. If there still is one, reaching out to AppleCare will be your best option so they can see if there is some account issue on the server end. The hardware is not what is at fault here.

I'm not sure why you're being charged for the activation messages for iMessage. This suggests that the phone thinks it's in a different country, from a different country or something strange.

Oh are you attempting to activate this with your carrier while you are traveling internationally? Is that why you were charged roaming charges? That would also explain why the service would struggle to connect if that is the case.

Was your previous device, the 6 plus, also a Cyprus/MTN device like your current one is?  Only asking to rule out the possibility that the 6 plus was activated elsewhere.

Next question: even though iMessage and Facetime are showing as not activated, do they still work over wi-fi?  Reason I ask is that I currently have a 4s that is locked to another carrier, has no sim in it (and so cannot possibly activate), yet the 4s works fine for iMessage and Facetime on wi-fi -- using AppleID and not a telephone number, of course.  As a last ditch effort, possibly test Facetime/iMessage by changing your number to your AppleID; that will at least eliminate the app as an issue.

Since MTN is definitely an iPhone compatible carrier, and your 6 plus worked fine on MTN, and you have tried multiple sims, and you've had your 6s replaced once and still have the issue, I am leaning towards a provisioning issue by MTN. 

The best test I can think of to test provisioning is to try your sim in another iphone.  Do you still have your 6 plus?  (though I am sure you have thought of that since you seem to have thought of everything else).

Hi

Go to settings second phone icon click on that at top of page make sure it is your correct phone number .

Cheers

Brian