>The plan is to equip them with some sms gateway software

MAXS (Modular Android XMPP Suite) could also work.

http://projectmaxs.org/homepage/

https://bitbucket.org/projectmaxs/maxs/src/master/

https://search.f-droid.org/?q=Maxs&lang=en

> Version 0.5.19 (2000506627) suggested Added on Jan 22, 2025

>This version requires Android 4.4 or newer.

(Hope the author is doing OK.)