I can tell you two solutions: one is to take a look to my post in the IOT section, called :
A BT Gateway between a smartphone and Arduino to control SMS-OT (like I-OT)
In this post I show how to use an old smartphone to behave like a gateway toward an Arduino.
The aim is to send SMS's to the old phone wich is connected by means of the BT to an Arduino board that actuates commands. In this case the old phone is in place of a SIM900: it allows you to control your home devices by sending one (or more) SMS to it.
The second is: I have written a code for the SIM900 and an Arduino board to remotely control the heating system of my country house (when we were still allowed to reach our second houses before the multiple lock-downs, here in Italy ) Of course I have written the user interface to send SMS's to the SIM900 in AI2.
Then, solution 1 is costless: you just take off from a remote corner of your desk drawer a forgotten phone and you give it a second life. Just retrieve the linked post and you have all: code and documentation written by me in English (my poor english :-))
The second costs you a SIM900 and a lot of tricky code on Arduino side.
If you prefer this second solution I can send you the AI2 code together with the Arduino code but in this case you shall be aware that all the comments inside the Arduino code are in italian language.
BTW in both cases the result is a remote controller for a (second) house heating system.
Let me know.