I have recently moved to UK from India, one of the initial things we do is get a local phone number.I got my new Sim and enabled iMessage and Facetime on my iphone 6s and it all worked like a breeze.
However, to check some of the messages from my India’s Sim, I had to put it back to my iphone. I checked my messages and then again replaced UK sim. After this, I was not able to enable iMessage and Facetime. It consistently gave error and even spending 2-3 hours with Apple Support on chat the problem was not fixed. I was not able to fix the problem even after disabling FaceTime and iMessage on other devices with same Apple id as suggested by Apple Support. Apple support ran out of options but nothing worked like reset the network settings etc , but no clue as I was receiving messages so network setting reset ended in no respite.
After few days, I noticed that I was not able to send messages, was just receiving them. My telecom plan did not allow me to send message, so I took another plan that enabled me to send messages. After that when I tried to enable iMessage and FaceTime, it worked. However, when I checked my topup balance it was reduced by 25P after enabling iMessage.
I learned that enabling iMessage may require your phone to send message that is done by the iPhone internally and Apple support was not aware about this, may be I was one of the unlucky ones to get a support person who was not aware about this.