Unfortunately all SMS/voice gateway are owned by paid services, thus there is no such thing as open-source, that I can recommend.

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.

个人学习可能取决于他人行为的主张突出了将学习环境视为一个涉及多个互动参与者的系统的重要性

is now in a sunset phase and will soon shut down

People, branding your new website require extensive efforts and time for initiating the activities, which will help to build the solid base of your site.  There are so many tasks needed to complete the overall branding as we all know that first impression is your last impression.  Online branding is very important for service providers because as a marketing guy, I know that it would be like stress somewhere the majority of the people are not known about the product.