It is working as it should when I connect to my ISP. I tried the problem just now. When I click on disconnect in kppp I am disconnected from the net . But when I try to use the phone,the line of which is being shared with the modem,I get a 'busy' tone not a continues dial tone from the phone.
This is only remedied if I close the linux session or I unplug the modem from the phone socket and put it back in again.