I can walk you through the steps of viewing a past bill with your previous phone number. Please follow the steps below:
Log in to your account online
Click on "My Account" at the Top of the page
Click on the "Billing" tab
On the right side of the page, click on "View Bill History"
Click on your "February" bill for example, and scroll down to the "Additional Voice Services" portion of the bill. Right below this header, you should see your old phone number - "For Telephone Number(s): YOUR OLD NUMBER HERE"
If you are unable to view this, please let me know and I can assist you further.
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