Fortunately, we found RingCaptcha (https://ringcaptcha.com), which has a the 'starter plan' that offers free 500 OTP monthly. Just a small plug for them for providing freemium service; they are highly reliable because they are integrated with all major global, and regional providers, e.g., Twilio, Nexmo, Infobip, MessageBird, etc., and send your OTP through the best provider/route based on country/phone carrier, and can auto fallback to alternative paths. This means you just need to integrate with RingCaptcha, without the headache of deciding which SMS/voice OTP provider has best combination of price and reliability, which is a real headache when you are sending OTP world-wide.

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