That's a great question and thanks for posting. Do you have a post paid account? If so, you can use the steps on our Check your usage page for a useful to check numbers that you've called in the past.
Did the info that Mike provide above help you find the phone number you were looking for? If not, do you have a prepaid account? If you have a prepaid account, your account usage is not available online and you would have to refer to the call log on your specific device instead.
I wanted to stop by one last time to see if the info that Mike and I previously provided helped you track down the phone numbers you were looking for. Please let us know if you have any other questions!