Textbelt uses email-to-SMS to send texts for free. The downside is that for some carriers the texts may appear to come from an email address. Delivery is theoretically unreliable, especially outside the US, because most but not all providers have these email gateways.
As a result I would recommend Twilio for actual businesses, but Textbelt is great for personal stuff and other limited use cases. There are many businesses and monitoring solutions that use Textbelt though. It just depends on your needs.
Textbelt uses sendmail under the hood and keeps a list of carrier email domains. So you're actually sending emails to 5552328491@att.com. Depending on the carrier, those text messages look a bit weird.
I checked the source to see what it's doing, and it actually is sending the text to every provider that has an email-to-text address, and therefore probably ignoring the returned failure emails.
As a result I would recommend Twilio for actual businesses, but Textbelt is great for personal stuff and other limited use cases. There are many businesses and monitoring solutions that use Textbelt though. It just depends on your needs.