External R factor for this call. This value SHOULD be calculated using ITU G.107 (The E Model) or extended versions thereof. The External R factor relates to the quality of an incoming voice from another network segment. For example if a conference bridge terminates and re-creates voice streams then an R factor would be calculated at the bridge for the endpoint A to bridge segment and relayed to the subsequent bridge to endpoint B as an External R factor. This allows endpoint B to estimate the end-to-end call quality.