callkit can interpret a hangup as a new inbound call

if A calls B, and B answers with callkit, then if B rejects the call, then A's callkit may start behaving as if it has an inbound call from B

Start by reproducing the reported sequence: A calls B, B answers through CallKit, and B rejects the call. Trace the CallKit call-state handling for both devices; done means rejecting B's call no longer causes A's CallKit to behave as if it received a new inbound call.

Written by the indexing model from the issue text.