Click the phone number on the left, then click [Delete] on the bottom right of the record. This will move the number into the cool down period. Repeat this step for all phone numbers on the account.
Next, go to Account > Settings.
In the top right is the account status button. Click it to expand the options, then select delete.
A popup will ask you to confirm you want to delete this account. If you’re sure, click [Delete] again to confirm.
Warning: Deleting an account is permanent. The account cannot be restored later, so be absolutely sure this is what you want to do.