Issues adding my phone number to my apple id after porting number from EE to Sky

I have recently swopped from EE to Sky Mobile. I requested a PAC code from EE and passed this to Sky so I could continue using my old number. It took a couple of days for the number to be transferred but I got an email telling me that the transfer had been successful. A day or so later I messaged a friend using imessage and he told me that it was displaying my temp number which I used until my old number had been transferred. I have turned off imessage and facetime and I have tried to add my number to my apple id account but it stating that 'Cannot Verify Phone Number, This phone number is not available. Choose a different number'. Because my number is not recognised I cannot received verification codes for banking & online shopping etc

I have done the following to try and resolve this

1. checked my time zone etc is correct
2. checked the number displayed under 'My Number' under 'Phone' under the settings tab
3. turned airplane mode off and on
4. turned the phone off & on several thousand times
5. tried turning imessage off and on but it still states that there is an error in the activation
6. spoken to Sky on several occasions but all they do is tell me to check that the correct number is displayed in the My Number section under phone and settings.
7. I have tried a network reset too and it is still not working

All I want to do is add my number to my Apple Id account which would then hopefully allow me to use the imessage services.

I'm slowly running out of options and I'm wondering if there is anyone who has had the same issue that has a resolution for me?