1. A fault isolation method for use in a network access server, the method comprising: allocating one or more modems in a group of modems associated with an access server to a debug modem pool; allocating one or more remaining modems in the group to a session modem pool; thereafter determining whether an incoming call to the access server meets predefined fault isolation criteria; and, if so, then allocating a debug modem from the debug modem pool to the incoming call and initiating a fault isolation session; and if not, then allocating a session modem from the session modem pool to the incoming call and proceeding with a normal session.