Of course, you don’t theoretically need a case for your phone. But if you want one, these ones at least do eventually break down (often only in industrial composting systems). This cute phone case is made from starch and cereals and composts in less than 3 months. Also in mint.
Making phone cases from recycled plastic is popular, but if these were dropped or lost at sea (say from a boat), they would still break down into microplastics, and get ingested by marine wildlife, so are not really that helpful, unless only used inside and guaranteed to never get lost.
This compostable phone case can be used with many Samsung phones. Made from wheat straw, it’s sent in a paper envelope with biodegradable glue on the postage label.
If you use your phone to play music at events, Gomi is a portable speaker made from 100 recycled plastic bags, powered by repurposed e-bike batteries. The two speakers create a wireless stereo system, to play music over a 10m range. The service includes repairs for life (and a portable charger made with reclaimed batteries.