I recently ported a Time Warner Cable number to T-Mobile then to Google Voice and learnt something that wasted a couple of days so I wanted to post about it to save others from wasting two days.
On my Time Warner Cable bill my account number consisted of three groups of numbers separated by dashes, (5 digits, 9 digits, 4 digits). The group of 9 digits were highlighted and they are your account number for porting purposes when you provide T-Mobile your information.
Everything else went according to plan as I ported from TWC to T-Mobile pre-paid, then to Google Voice. As others have mentioned the port to Google Voice is a 24 hour process almost to the minute.