(Set your phone background here then use this)
set format phonetext
@CHARACTER left (the character you want to appear on the left)
@CHARACTER 1 right (the character you want to appear on the right)
Then you just use normal dialogue coding for the texts e.g:
CHARACTER
Do you want to go shopping?
CHARACTER 1
Sure, when do you want to go?
When you want to set it back you do:
set format cinematic