Hey Rick - I can't resist an "I told ya so". If you are just concerned about cost but you still want to use an API for SMS through a transaction broker, Google for "twilio competition".
But solution providers and businesses need to evolve with the times. We can't look forward to technology of the prior decade. That's what kills a lot of MV developers.
I've been working on multi-channel messaging that delivers messages to consumers in whatever medium they prefer. Example of channels include Twitter, SMS, Slack, Skype, Discord, Telegram, Whatsapp, Instagram, Facebook Messenger, and Email. We can also send messages to IFTTT and Zapier, which themselves support a huge number of channels and applications. By supporting channels like this we don't need to tell people how they're going to get our messages anymore. We offer notifications and let the consumer decide when and how to get them.
This should be considered an AD because I would
welcome sponsorship for efforts on this project, projects with clients for similar integrations, and partnership for the
resulting offerings.
One "simple" (not really but sorta...) way to support messaging is to create a cross-platform mobile app with Xamarin that does nothing but open a notification for messages queued on a server. It's completely free, there are no limits. Your audience is limited to Android and iOS users.
That gets you about 97% of the market that has a device, and that includes Wi-Fi-only devices that you didn't reach with SMS. For the other 3% on "dumb" devices you can still send SMS or email and it won't cost you nearly as much as what you've been concerned about spending. Obviously you have already written off those who do not have a device/service for SMS. Whether you write off that other 3% depends on the business model.
HTH
T