One of RingCentral's most popular features is custom fields. However, this feature is often not enabled by default.
If you are receiving the following error:
{
  "errorCode": "BIL-103",
  "message": "Feature [CustomFields] is not available for current account",
  "errors": [
   { "errorCode": "BIL-103",
    "message": "Feature [CustomFields] is not available for current account"
   }
  ],
  "feature": "CustomFields"
}
Then the custom field feature needs to be enabled for your account, which can be done free of charge by our support team: