Add proposal for "completed elsewhere" hangup reason.

proposals/4006-voip-completed-elsewhere.md

@@ -0,0 +1,47 @@
+# MSC4006: Completed Elsewhere for VoIP
+
+Usually, it is self-evident when a VoIP call in Matrix has been answered by a different client:
+the answer event from a different party ID is present in the room. However, bridges can complicate
+this. If one client is SIP bridge, for example, the SIP server will just send a CANCEL on any call
+forks that are not answered, adding a cause of 200 with text, "Call completed elsewhere"
+(https://www.rfc-editor.org/rfc/rfc3326). This cause code will currently be ignored by the bridge as
+there is no equivalent to translate it to, so the call will show as a missed call.
+
+## Proposal
+
+Add another value for the `reason` field in an `m.call.hangup` event of `completed_elsewhere`.
+
+```
+{
+  "type": "m.call.hangup",
+  "content": {
+    "call_id": "1234567",
+    "party_id": "AAAAAAA",
+    "version": "1",
+    "reason": "completed_elsewhere"
+  },
+}
+```
+
+## Potential issues
+
+This would make the VoIP spec marginally more complex for all clients, when a large majority of users
+are probably unlikely to interact with such a bridge, let alone complete a call elsewhere. However,
+if clients choose to ignore this reason code, the failure mode will simply be that the call shows as
+a missed call (as it does currently).
+
+## Alternatives
+
+This could potentially be done without extra spec if the bridge were to create a fake answer event.
+However, in SIP, the bridge would get no further information about the call, so wouldn't know when it
+ended, meaning that it would have to send a fake hangup straight afterwards, so the call would appear
+as zero duration.
+
+## Security considerations
+
+None forseen.
+
+## Unstable prefix
+
+Until merged, the reason code shall be, `org.matrix.msc4006.completed_elsewhere`.
+