I've got an old Audiovox SMT5600 that my wife uses. Since the beginning of July or so we have been getting errors on, when the phone is not even being used, that say:

Connectoin Error
Unable to place call

When we do use the phone to make a voice call, it says something like:

Unable to place data call while in voice mode

Well now it turns out that data calls of some kind must have been made because we have been billed over $350 US for data on that phone even though we NEVER use the internet on it. My wife does not even know how.

One thing that she remembers is getting a MMS message with a photo attached...the photo never came through. When trying to get the photo to download that's when she remembers the error messages starting.

Is it possible these are MMS data calls trying to download the photo over and over for 2 months now?

ATT will not help and thinks we were just running up long internet sessions on the phone.

Help!!!

Sounds strange, when I have had a failed MMS message I have always had to physically choose to reattempt a download.
What may be the cause is that perhaps your wife was not locking the phone's keypad and so while it was in her handbag/pocket etc an internet session may have been started without her knowledge?
I know my wife never locks her phone and then complains when the youngest has a play and dials out.

Only a suggestion but it's all I can think of as there are no known viruses on the WM format that I am aware of, unlike Symbian which did have a little virus that dialled out without the users knowledge.