Busy flag and Dial tone

A call progress detector would do the trick, such as an MX663:

I just grabbed these examples in a quick search, you want to see what is available once you know the key terms to search for. A modem chip may be more easily obtainable than a call progress detector, though the latter would, at first thought anyway, be easier to program. In a pinch you could get a PIC with a modem built-in and use that.

Ken