[Serusers] Failure route not working with cisco

Hi After a couple of weeks of stress, I have managed to get pstn connectivity working fine, to this I am trying to add voicemail with asterisk. Now when i send out a call to the pstn via cisco, what I want is that if the call is not answered, or abusy signal is sent, that the t_on_failure section kicks in and we are forwarded to asterisk. My pseudo code is as follows If uri starts with 9[0-9] then send to route 3 route 3 { if method = INVITE { forward call to cisco } } Now when the call is not answered, I seem to get method=ACK messages in my logs, so when the conf file is ran through again obvioulsy route 3 is hit, but no match cause the first statement is looking for a INVITE, I have added the ACK method also, and then it works fine. What I wanted to know is this the norm, and is the method of adding the ACK there okay, or is it a kludge which will break if I change my UA, or change my cisco box. If I cancel the call, then the method is <CANCEL> which is okay, but I just cant see where the ACK method comes from if call not answered tks iqbal