A two-bin kanban system is simple on the shop floor: when a bin is empty, its card goes back to purchasing and the supplier refills it. The weak point is the step in between. Cards collect in a box, someone types part numbers into an email or reads them out on the phone, and an M6 turns into an M8 on the way.
Suppliers who refill bins on site or by consignment have the same problem from the other side. They need a clean list of which bins are empty, with their own part numbers, not a photo of a handwritten note.
If every bin already carries a barcode label, the list can be built by scanning the labels. The phone shows the part name for each scan, so a wrong card is noticed at the bin, and the finished list goes to the supplier as a file attached to an email.