Any calls we receive externally, if not answered hangs up after 5 rings. If we call the number internally works fine, the phone continues to ring. If the first line is busy it moves to the next line in the hunt group. We do use a phone tree to access the different departments. The setup has been working for the last 14 years, we have made no changes to the system. Any help would be appreciated. We are using Mitel 3300.