Around November 2006, there was a change in policy where only people listed on the accounts are able to access the account. This was done to increase security of customer's accounts, and prevent unauthorized changes. It seems like this change occurred between the times when you called to disconnect the cable and internet, and when you called to try to cancel the phone. This is why you were not successful when attempting to disconnect the phone. You simply were not authorized. Obviously situations do arise where customer's do pass away, and leave nobody listed as authorized on their accounts, but there are ways around this, ie. Death Certificates. I'm sure this was explained to you when you called in, probably every time, but i can see why you would leave it out of your complaint, because it makes your post seem less valid. :)
It seems to me, this situation would have been resolved much sooner had you just brought in a copy of the death certificate when this was suggested to you. I'm not really sure why you would not have done that, as it would be much easier than calling in over and over, and writing letters to the corporate office.
I personally feel more secure knowing that a person cannot randomly call my phone company, tell them I'm dead with no proof, and have my account shut down. Don't you?