It's a difficult one for sure @kmcurso89
There are a couple of options for you to consider.
Change of Ownership Probably the best option. This will cover the airtime part of the contract for the number your ex used.
Cancel Account Follow the guide through, as you have more than one number this can be completed through your online account. This is your account you are free to cancel or transfer the number your ex used to any other network.
If there is a credit agreement in place for the device this will be in your name and separate and you will need to pay it off or continue with the monthly repayments, it's won't be something that can be transferred.
