Best SMS Service Provider for Global OTP Verification and Phone Number Validation (Any Free Options?)

I'm implementing OTP verification for user signup/login in my application and I need a reliable SMS service provider. My main goals are:

Global Reach – The service should support sending SMS OTPs to users in multiple countries.

OTP Verification – Fast and reliable OTP delivery is essential.

Phone Number Validation – I’d like to validate if a number is real/reachable before sending the OTP.

I’ve found a few providers like:

Twilio

MessageBird

Nexmo (Vonage)

Firebase Phone Auth

D7 Networks

My Questions:

  1. Which SMS service is best for global OTP delivery in terms of reliability and cost?

  2. Are there any services that also provide real-time phone number validation (e.g., like Twilio’s Lookup API)?

  3. Are there any free or low-cost SMS services or workarounds (especially for development/testing)?

  4. Is it worth using a separate phone validation API before sending OTPs?

  5. Besides SMS, are there any alternative methods to verify a phone number (e.g., missed call verification, WhatsApp, flash call, etc.)?

If you’ve had experience with any of these (or others), I’d really appreciate your recommendations.

Thanks in advance!

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