That's the opposite of what most people want 🙂
I dunno, I'm stumped. Check the settings in Modem Properties, and make sure the drivers didn't add a 70# setting to disable call waiting on each dial.
Other than that, there's not much you can do besides buying one of those "call waiting modems". The problem seems to be that your LT winmodem is ignoring the call-waiting signal and treating it as line noise.
Modus