I was recently travelling through Honduras and came across the situation when a decal / sticker ended up on the back on my iPhone (well actually it was young niece who asked which Dora sticker I would prefer).
So I have a great iPhone, though with a sticker on the back that has only half come off. I tried initially peeling it off, though only the shiny sticker part came off. What was left was the adhesive and some rough paper.
I was cautious of using any spirits or chemicals to remove the sticker as the back of the iPhone is plastic, so didn’t want to damage the surface. Hence I decided to use some Eucalyptus oil.
Good stuff – the adhesive easily rubbed off and my iphone is looking cleaner than ever
(P.S – I haven’t told my niece that the Dora sticker is no longer on the back of my phone)