You are not really helping me a whole lot to solve your problem. You providing more information would help your cause. How many phones? What it sounds like to me is that phone is losing heartbeat to the 200ICP controller. As I asked you in the first set of questions is what is different about that phone? Is this the ONLY 5312 on this controller? Have you looked at the logs in your 200?
You should be VLANing inorder to isolate the voice traffic. If someone is listening to internet radio, downloading a big video file or something it will drag down the network and not allow the phone to communicate with the controller as often as it wants to, hence dropped calls.
Is the computer connected behind the phone or is it on a seperate drop?
