Modem disconnect after 15 min inactivity

I am supporting a site running NT, Proxy Server 2.0 & Exchange Server 5.5 on the same box. A Netcomm Wave SP56 modem (ex Banksia) is used to connect to the ISP. The ISP modems are set to disconnect after 15 minutes of inactivity. Apart from signing up for a permanent modem connection with the ISP is there any other way to keep the link open?