If you're wondering which SMS API provider would be the best or the most reliable for your startup to send OTPs during user registrations or signups, I would highly recommend Msg91.com
I have been researching for a good SMS API service that not only has great documentation but also lives up to the expectations and send the OTP within milliseconds. I found a few but it doesn't come in the same package like great OTP sending speed or the cost or the documentation. MSG91 however seems to do all that.
Our tech team who hasn't done any OTP integration before did it within 30 minutes of going through the documentation. It was well explained!
They provide 100 OTP SMS for free in the beginning to test and work it out which is great. They cost of the SMS package is also very minimal. If you have a startup and a working prototype, integrated with MSG91, you may apply for their Startup Plan too which gives free credits.
Rest assured, I highly recommend them for your SMS API integration needs.