Founded in 2009 by Jan Koum and Brian Acton and acquired by Meta for $19 billion in 2014, WhatsApp has more than 3 billion monthly active users with default end-to-end encryption and WhatsApp Business and API services.
Brevo (formerly Sendinblue) is a French all-in-one marketing platform integrating email, SMS, WhatsApp, chat, and CRM. Free plan offers 300 emails/day with unlimited contacts.
Twilio is a cloud communications API platform (CPaaS) founded in 2008 in San Francisco and listed on the NYSE in 2016, covering 180+ countries. SMS is about $0.0079 per segment and voice about $0.013 per minute, with SendGrid acquired in 2019 and Segment in 2020.