Just out of curiosity, if we get a duration shorter than permanent can we specifically summon the same devil again for the next try?
Since the fluff from the chapter mentions them gaining names, and we might either want to retain a character or avoid letting a devil who's had access to our stuff stay in the wild.
Yes, you can also keep the charm on anyone that is still there.
Most Spirits aren't in the same plane of existence as a Yama King, under their direct rule, with self serving minds.
This is like the difference between having a friend who lives in town, and having a friend who is a deep cover spy against literal Hell.
The spirit on your phone no matter its nature could be tortured into spilling your secrets if someone got their hands on your phone.