Hey! I don’t know if you’re still looking for it, but I was digging around myself because I was also looking for the solution to this. I found the reddit post by Broken__Glass here:
https://www.reddit.com/r/RenPy/comments/1fu6dwl/yet_another_phone_and_adding_choices/
But you may also want to look at this forum post if you want to add dialogue choices to the phone:
https://lemmasoft.renai.us/forums/viewtopic.php?f=51&t=62837#p550774
Scroll down to chekkin’s comment and you should find what you’re looking for there!