Hi, sorry for the late response; I just answered a similar question above:
Let's imagine you have a prologue, act 1 and act 2 and you want a different phone background for each of them.
You can have a variable "chapter" that will tell us where we are and change during the game:
$ chapter = "prologue"
And we can then edit the script in PhoneTexting.rpy to have a dynamic background instead. In line 158:
style phoneFrame_frame: background "phone_background.png" foreground "phone_foreground.png"
Replace the background by something like:
style phoneFrame_frame: background "phone_background_[chapter].png" foreground "phone_foreground.png"
This mean that Renpy will automatically look for "phone_background_prologue.png", "phone_background_act1.png"... etc whenever you change the value of chapter!
Make sure to have the image created in your project and correctly named, and you should be good