Following the Google Voice step by step instruction won't have you phone number ported to Google Voice. And Google help tells you that only Spring prepaid can be ported. Here is some manual instruction that you can port your number to Google Voice.
- T-Mobile account number
With prepaid plan, you won't receive any bill or have bill summary in your account management page, so you won't know what your account is when Google needs you to provide account number in the number porting form.
The account number is actually your prepaid phone number with prefix "1". So for example, if your phone number is "669-123-4567", then your account number is "16691234567"
- The account PIN
You PIN code is also required if you got the error code saying that PIN code is incorrect when number porting. This is different from your voicemail PIN code. You can set your account PIN or bill PIN through dialing "611". Then say "manage my account", and then say "change my account PIN", and say or press 4 digitals to set your account PIN.
So when Google Voice prompts you that PIN is incorrect and you have to update the information, use your new account PIN.
So anything else in the number porting application form is required but not to be correct, such as billing address, you can fill your own address for example. Then your T-Mobile prepaid number should be able to port to Google Voice within 24 hours now.