You must fully port the land line number to a prepaid carrier. That means, you must put the SIM in a working mobile phone handset and activate service with the mobile carrier, and pay them the minimum amount for a bucket of calling minutes, so that the phone can make and receive telephone calls and send and receive text messages. If you don't do this, the port into Google Voice will be impossible. Don't be a cheapskate. After all, you are basically wasting T-Mobile's time and resources by porting in and immediately porting out. Consider this compensation to T-Mobile to do that.
Wait at least until next week before attempting the subsequent port into Google Voice. Do not initiate it over this weekend. Attempting to immediately churn a port will fail.