Advice needed on SMSing

I am starting a store which allows a purchase when the customer has not provided an email but just a mobile phone number. In that case I am supposed to send the customer a SMS. For technical reasons I cannot use mail-to-sms service. I prefer to use the API of gatewayapi.com. The platform is nop 4.30.
My question is: What is the best solution in my situation? Is there anything ready-to-use or I should make my own implementation of IWorkflowMessageService?
It is not an easy question. Shopify has similar mail-or-sms feature. It would be good if we have it too in our open source. Big players as linkmobility.com are providing a service that retries messaging to email, viber and SMS in the given order. But that service is minimum €200/month which is small business killer. Any thoughts on that?

Not that I have used any of them or can recommend the cheapest but there are a few free plugin options to investigate
There is https://www.nopcommerce.com/en/sendinblue which provides SMS Notifications and is included in the core system
Also a few other plugins
https://www.nopcommerce.com/en/extensions?searchterm=sms&version=430

This plugin offers the one of the best SMS integration in nopCommerce including OTP on registration and you can extend this plugin easily using your custom methods.