If I was aware the process to port a number from Belong to TPG was not easy I would not have attempted.
This is what it says on the Belong website: If you're trying to port your Belong mobile number to another provider and it hasn't worked, try double-checking your Belong account number (just select the button below to head to the 'invoices' section). If you've got the right account number and it's still not working, please get in touch with your new provider for more assistance. You will continue to be charged for your Belong mobile service for each monthly payment cycle until your mobile number is ported or Belong service is cancelled.
So I end up up going in circles between TPG and Belong.