what is the frequency of call waiting ?
440Hz. although i am not sure if it's a standard or not. there is a link below which elaborates more on it. http://www.leinweb.com/snackbar/internot/
how can i detect call waiting tone ?
i would use a 567 tone decoder ic. i've used them in the past, and they've never failed me. they work on 5V, use minimal components. you can also have the ouput of the ic latch if you like.