My cellphone carrier doesn't offer a free API for programmatically sending SMS messages. Or at least not for free, they do have an option for businesses but it's extremely expensive.

I have an old Android phone I could keep in a drawer with a SIM card to act as an SMS gateway. My question is: Is there a service that allows me to expose an HTTP REST endpoint for (securely) sending (maybe receiving too) SMS messages? Better if it's FOSS. Thanks!