A system for assuring quality-of service for Data Network Telephone (DNT) calls, such as Internet Protocol Network Telephony (IPNT) calls has software for monitoring established DNT calls for adequate bandwidth. A threshold bandwidth, or parameter value related to bandwidth is stored, and tests are periodically made on the DNT call path and compared to the threshold. If the available bandwidth falls below the threshold, an alternative conventional call, such as a PSTN or ISDN call is made to replace the DNT call. Once the alternative call is established, the original call is ended. After the original call is ended, tests of the DNT path continue, and, in some cases, if adequate DNT bandwidth becomes available, the call is switched back to DNT. In some cases, alerts are posted for call participants, and provision is made for approval by one or both participants in a call for a switch in service type to be made.