Twilio - Brings voice and messaging to your web and mobile applications.
Okta - Enterprise-grade identity management for all your apps, users & devices
Bandwidth - Bandwidth offers SIP trunking, Emergency communications, and Voice & Messaging APIs.
OneLogin - On-demand SSO, directory integration, user provisioning and more
Nexmo - Nexmo is a simple two way SMS API with global reach and wholesale rates
Auth0 - Auth0 is a program for people to get authentication and authorization services for their own business use.