SenderID or Header registration is a mandatory process for clients whose business is registered within India and who wishes to send messages within India
The DLT registration process can be found here, for your guidance, across all operator platforms.
Once the Company (Principal Entity) as well as the SenderID is registered, you would need to go ahead and register the same SenderID on Kaleyra's platform as well.
In case you’re wondering how to do that, you can find out here.
Trying to send messages from Kaleyra platform with DLT unapproved SenderIDs or Headers will result in the messages not being sent to your clients.