You will need to obtain a NNID (NetNumber ID) from NetNumber directly.
To add a new NNID under Sinch, we need to submit a form to T-Mobile and request provisioning. The cost is $2k USD which has been waived until further notice.
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!