FAS is one of the common problem in voip network. there could be lots of point from where this issue is raising, it could be low rate carrier or you must be using routes for destination which does not comes under our POP network. We fight FAS as soon as we get report for it at any moment we cannot assure that any given destination could be 100% FAS free.
Please make sure that you are not getting dead air. if you raise FAS complain ticket over dead air then after testing we may close your ticket since both of them are separate issue.