You will need to obtain a NNID (NetNumber ID) from NetNumber directly.
To add a new NNID under Sinch, the NNID needs to be provisioned with the carriers. An LOA and form must be submitted to T-Mobile to request provisioning. The cost is $2000 USD.
Visit the Sinch 10DLC Product Page to learn more or if you need more help with 10DLC or have any questions, contact Sinch, who will be happy help!