Before revision: The Mitel Superset Series of Office Telephones The Mitel Superset 4DN Telephone The Superset 4DN was one of the first Digital Telephones made by Mitel
After revision: and was announced in October 1986.The Mitel Superset Series of Office Telephones The Mitel Superset 4DN Telephone The Superset 4DN was one of the first Digital Telephones made by Mitel