I had a 3g modem in my cisco router that I used to use for that, but when they shut down the 3g network I was never able to find another cheap sms-only service for the 4g version of the cisco modem. (So I switched to wifi & xmpp.)

I wrote my own software to use it, talking to the raw modem interface. Which, interestingly, uses an extension of the old “AT” modem command set. Weird.

Being able to write shell scripts that access sms is fantastic, I miss it.