1. A method for assuring quality of service (QoS) for data network telephony (DNT) calls, comprising steps of: (a) monitoring bandwidth on an existing DNT call between two stations; (b) comparing available bandwidth to a prestored threshold value; and (c) in the event that available bandwidth is less than the prestored threshold value, establishing a conventional telephone call via a telephony channel between telephones available to the participants in the DNT call, and terminating the DNT call.