Wrong phone number in iphone imessage

Hi. I have an iphone 6+ and I'm having trouble activating imessage.

In settings >Messages> send and receive> I can see my email (which is my apple ID). As I click on it it will show:

You can be reached by imessage at:

my OLD phone number (greyed out) and my correct email (the email is checked but he phone number isn't, but I can't change it because it's greyed out).

Before you post your much appreciated help, Please take into account the following:

I do not have the old SIM card (the one whose number is showing up on imessage (greyed out). I only have my new (actual) SIM card available.

The only place where my OLD phone number shows up is in my iphone's imessage. I have checked my apple ID and my phone number is the correct one. I have also checked Settings > Phone > My number: It shows the correct phone number. I've tried removing the country code and rebooting to no avail. With or without country code it doesn't make any difference whatsoever.

I have tried the times the activation button: Your carrier may charge you for SMS.... etc. both before and after airplane mode. Turning iphone off and back on, respringining, rebooting. (BTW I can send international SMSs).

Of course I have tried a dozen times all the procedures explained by users in various forums, including airplane mode, removing sim card, signing off and on from apple. Changing phone number in apple ID from my computer and from my iphone.

As per facetime, the story is exactly the same.

Conclusion: No imessage, no facetime. Wrong phone number so far impossible to remove from imessage.

Voilá. Your help is much appreciated

M