Tunde,
There are many reasons why the calls could break. You need to try to troubleshoot this down to the side that could be causing the problem. One big issue could be the internet connection. You should check to see how stable your internet connection is between your Tenor and the other side of the IP call. A simple ping test from a PC located on one of the networks and ping to the other IP network can tell you if the network is stable. You may want to try using pingplotter (www.pingplotter.com) as this can provide more information.