Here is the information that we have compiled over on the Google Voice forum, with regard to T-Mobile prepaid. This is unofficial, and since carriers change things periodically, I can't guarantee that it is accurate, but it's highly likely to work:
*PayGo "My T-Mobile Account" shows your account number. It is the number on the top right of the screen (9 Digits). It also shows it under the Account Activity Log.
*Legacy Pay as you Go accounts = your 10-digit phone number with 1 in front is your account number
PIN - 4-digit PIN....To set a PIN, dial 611, say "no" to refill, choose "manage my account", choose "change my account pin", then set your 4-digit PIN
Customer Service: 1-505-998-3793
Prepaid Porting: 1-877-778-2106
The porting form that first appears, along with some T-Mobile-specific instructions, does not include a field to enter the PIN.
After submitting the form, you will get an automated email note within the half-hour, complaining about the missing PIN or SSN.
Go back to the form, and at that point, it will display the fields.
Enter the PIN and a dummy SSN, and resubmit.