Phone Line Utilities

Can anyone recommend me a program (preferably free but not necessarily) that will allow me to use a modem to query a phone line and return a valve based on whether or not the phone line is engaged? I want to integrate it into a batch file to check that if the phone line is not engaged and the dial a divert string on the line at 17.30 each evening and then dial the cancel divert string at 7.30 the next morning. The timing can easily be done with batch files is just the line status checking I can’t work out.
Ideas please people.
Cheers

Steg