Did you try InternetAutodialHangup() API call?

DECLARE INTEGER InternetAutodialHangup IN wininet.dll INTEGER dwReserved  
=InternetAutodialHangup()

Check also Dial-Up Connection Does Not Hang Up mskb #Q235666.

>I have a situation where the modem is dialed up to the internet. I did not initiate the dialup connection. But I need to hang up the modem.
>
>Does anyone have any VFP code that will hang the modem up? I've tried several things but I can't seem to get it to work.
>
>Thanks,
>Trish