Remote modem keeps dialing 1800

If the modem is outside of your control, then you are pretty much stuck. Even if you solve this one, the 1800 person is going to have problems often as people forget the "9" when making a toll free call. I often wonder if we should just give up and never assign extension that start with "1"

If you can capture the caller ID, you will at least have a start at finding out what person or company owns that modem.

it's probably one of your fax machines. if this is a constant thing forward