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What happens if the line is busy or there is no answer when receiving a text-to-speech call?

In the case of the text-to-speech (TTS) call doesn't go through, a “DELIVER_OTP_RES_FAIL” event will be reported. The OTP will remain for the duration set by the customer (default is five minutes) — and can be resent again. You are not charged for these additional deliveries/delivery attempts. Check out more details here.