This service does not use a commercial SMS API.
Instead, I have set up a phone with a physical SIM card that is running a custom app to forward SMS messages to the server running this application.

Said phone has been firewalled from the Internet, and mobile data is turned off (on top of that the SIM does not have a data plan so I would be burning money by enabling data).
The only destination outside of its own network (my home IoT VLAN) that the phone can talk to is this server.

Since this is just running on a phone, do note that I may manually see any text messages you may send to this phone and the numbers that they have been sent from.

Feel free to use disposable SMS services.
I have not placed any explicit restrictions on such providers.

Source code coming soon.