The REST API documentation for the command /api/v1/groups/{GROUPID.EN_US}/external-communication describes the PUT request as a way to update the existing settings. However, you are not able to update the settings, only add or replace existing settings.

The REST API documentation can be found on the Symantec Endpoint Protection Manager using the following address:

https://SEPM-IP:8446/sepm/restapidocs.html

Alternately, see Symantec Endpoint Protection Manager 14 REST API Reference.

The documentation describes the PUT request as "Update external communication settings to a given group" when it should read "Add or replace external communication settings to a given group." The documentation is slated to be updated for a future release.