TS2755 imessage displays wrong cell number

Hi

I have an iphone 5 operating on ios 6 through t-mobile in the UK.

I have changed mobile operator and thus have transferred my old number ending 064 to the new account, ending 817.

The problem is that my imessage account displays the 817 number and not my 064 number.

I do not have the option to change the 817 number in my send and receive settings as it is ticked and greyed out.

when sending a message the 817 number is displayed and none of my contacts know this number.

Can you please advise how I change this number to 064?

thanks

S