Using SMS Credits is related to using the SMS Gate feature.

SMS credit allows you to send messages to mobile numbers. For now, it is reserved for sending verification codes after a viewer unlocks a video by entering their phone number via the SMS Gate feature. This way you will safely lock your video and allow access only to viewers which provide a valid mobile phone number.
You can check your current credits and buy credits by navigating to Account & Billing >> License & Consumption:

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