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). 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. Also see how to switch to a greener phone company.
Blue Marigold Printed Eco Phone Case is made from bamboo and bioplastic, and will fully biodegrade in six months. The printed area does not cover the whole phone, just the back. Also in floral design.
Biodegradable Airpods Case
Uunique Biodegradable Airpods Case is made from plants, and offers shock-proof protection. In six colours, inspired by nature.
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.