If they're using the escape code "hack" (+++ATH0), you can prevent that on putting guard time on the espace code, the +++. This means there has to be 1 second (or another specified time) of nothing before and after the +++ is sent, else it won't go into command mode where they can give hangup commands or anything else. This is a very good protection because it's almost impossible to do remotely. Please check out this page:
http://www.modemsite.com/56k/escape.asp.
Basically that page says that you can disable the escape code completely by putting in s2=255 in the extra settings for your modem (like surfi said), but the guard time is a better way, by putting s12=(time you want in 50's of a second for guard time) in the extra settings. I'd make it one second.
Good luck
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