message: Prepare to encrypt when a call arrives
  So we're ready to send an answer straight away if the user answers
diff: @@ -681,6 +681,12 @@ export default class CallHandler {
  Analytics.trackEvent('voip', 'receiveCall', 'type', call.type);
  this.calls.set(mappedRoomId, call)
  this.setCallListeners(call);
  +
  + // get ready to send encrypted events in the room, so if the user does answer
  + // the call, we'll be ready to send. NB. This is the protocol-level room ID not
  + // the mapped one: that's where we'll send the events.
  + const cli = MatrixClientPeg.get();
  + cli.prepareToEncrypt(cli.getRoom(call.roomId));
  }
  break;
  case 'hangup':
  