What is the ruling on wearing a phone ring on my middle or index finger, by analogy with the ruling on other rings?

What is attached to the back of a phone for easy gripping is not considered a ring, and therefore there is no objection to wearing it on any finger. The prohibition of wearing a ring on the middle finger and the one next to it is a prohibition of karahat tanzih (disliked but not forbidden), not tahrim (forbidden), and the Sunnah is for the ring to be worn on the little finger.