GO BACK TO XFINITY FROM T MOBILE

I MADE THE MISTAKE YESTERDAY OF SWITCHING TO TMOBILE AND NOW I WANT TO SWITCH BACK TO XFINITY. WHEN I GO TO ADD A LINE IT SAYS YOU CAN'T ADD A DUPLICATE NUMBER.

Sorry for the caps......We have 2 numbers, one which was already switched to t mobile yesterday, the other remains with xfinity, so i want to add the other number back to xfinity.

how can this be done?