Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Google Authenticator - Google Authenticator is a multifactor app for mobile devices.
MessageBird - Reach 7 billion phones in seconds via SMS, Chat & Voice. Try it for free and improve your communication.
andOTP - andOTP is a two-factor authentication App for Android 4.4+
Plivo - Plivo simplifies your customer engagement.
Microsoft Authenticator - One app to quickly and securely verify your identity online, for all of your accounts.