Help - modem not recognising Busy signal

I have a Conexant 56k modem and it cannot
recognise the busy line witch makes me crazy!
All my prior modems, even in 9600/14.4 era, knew the busy
signal without any tweaking.
I tried the X strings from 1 to 4 but with no success. I
guess there has to be some S string, maybe a sound (hz)
tweaking, or some other command that I haven't tried...
Please help, it's really annoying!!
Windows is XP Pro.