Average Hold Time, or AHT, refers to the average length of time between the moment a caller finishes dialing and the moment the call is answered or terminated. The formula for measuring AHT is the total call time divided by the amount of calls. To achieve a realistic AHT figure, the ITU recommendation is that the AHT calculation be based on trace samples ranging from 15 to 60 minutes.

International Multimedia Teleconferencing Consortium, Inc. (IMTC)
Home page of IMTC, a non-profit corporation dedicated to promoting and facilitating the development and implementation of interoperable multimedia teleconferencing solutions based on open international standards.

Modelling Approach for VoIP Traffic Aggregations (PDF)
This paper gives a theoretical approach to model an aggregated flow of VoIP connections on packet basis which is created due to tele-traffic parameters.