Low ACD could be for many reasons. Wrong codec is not one of them. If the codec is wrong, the call will fail completely. Some reasons for low acd could be misconfig on answer supervision, a wrong hop off pattern, poor quality IP and/or phone line that causes people to hang up.
The best thing to do is to look at your CDR, pick a couple of calls that have low acd and call through the system yourself to those numbers to see what you hear.
As for the signaling, the only time you will get these messages is when a call is set to you ro when thte call is disconnected. Signaling messages are not sent any other time and the amount of bw is extremely low for them, not even worth mentioning. One thing you can check on is to make sure that your provider is never sending you more calls than your unit can take. Make sure that they look at the number of calls in progress and not connected.
If you would like some additional help in troubleshooting this problem, you can contact me at mike_voip@hotmail.com and we can discuss your issue and my services.
Mike