You will not be charged extra if you try to send SMS to any DND number(s). You can only send transactional messages to DND numbers or if the recipient of such messages is registered to your company/services and has opted in to receive SMS from your company.
Promotional or Marketing messages are strictly prohibited from sending to DND numbers.
However, as per the new directive by TRAI, if and when a message does get scrubbed at the DLT platform level and then fails due to "PREFERENCE NOT MATCHED" status because the recipient has chosen not to receive promotional messages specific to a certain category, then these messages will be charged for the scrubbing charges of 2.5 Paise/message.
Kaleyra would not levy any extra charges on this.