The Direct Carrier Billing API does not expose the user's MSISDN but uses an alias instead. The alias remains stable per user and per service, so the service provider will know whether the same user wants to subscribe to the same service multiple times.
If the user is using his mobile network internet connection at the time of validating a payment or a subscription, he is automatically and transparently recognized by the operator and no additional identification steps need to be taken by the service provider or the user.
If the user however uses a Wi-Fi connection, he is automatically redirected to a SMS Link flow. This requires him to enter his MSISDN into a web form, after which he will receive a one-time link via SMS. Clicking on that link will validate the user's identity and forward him to the payment or subscription confirmation page.
This entire process is automated from the service provider's perspective.