Removes a user's or group's access grant on a phone number. A number must always keep at least one owner, so removing the last one is refused with 409. Removing a grant that does not exist still returns 200. The change is recorded in the number's history. An Admin key can manage access on any number on your account; any other key must itself be an owner of the number, and a key's grants are its own rather than those of the person who created it.