Google Voice is not a phone company. It can't provide mobile phone service to your mobile phone.
Porting is a telephone industry procedure. With your permission, the "gaining" carrier asks the "losing" carrier to take over providing service for your phone number. After the losing carrier approves, and the gaining carrier takes over the number, service on that number with the losing carrier is canceled. So, in this case, after you port your now-T-Mobile number to Google Voice, your mobile phone will stop working, its SIM will be dead...throw the SIM in the trash. The number is now controlled by Google Voice.
If you want mobile phone service, you will need to obtain it from a mobile phone carrier.