Twilio - Brings voice and messaging to your web and mobile applications.
andOTP - andOTP is a two-factor authentication App for Android 4.4+
Plivo - Plivo simplifies your customer engagement.
Aegis Authenticator - Aegis Authenticator is a free, secure and open source app to manage your 2-step verification tokens...
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
OTP Auth - The app for calculating one-time-passwords on iPhone and iPad.