Code SMS notification for website | Proxies-free

I recently worked with a client on the landing page project. She wanted the opportunity to accept the e-mail and phone number. Once the customer has completed the form, there should be an e-mail and the SMS generated regardless of their country. I understand that you can use the e-mail responder for the e-mail feature. Such as mailchimp or the Blue Services. But if I want to code the SMS notification option, what is the opportunity for me as a programmer to do something like a project?

How did you implement the SMS notification service in the landing page or in any project?