callOrigin was set to event.callOrigin (the HOST instance URL), which
routed accept/reject through the multi-instance WS connection. On
mobile hotspot or when the multi-instance WS drops, the accept is
silently lost — the host never knows, the call stays ringing forever.
Now callOrigin = origin (the WS that delivered dm_call_incoming).
This is always connected. The server on that instance finds the
FederatedCallEntry and relays to the host via S2S HTTP, which is
reliable and independent of client WS state.