System Standby Disconnects Modem Connections

When you place a Windows XP-based computer on standby, any currently connected remote access connections are silently disconnected. You are not prompted to disconnect manually before the computer goes on standby. This applies to any remote access connections, including connections to Internet service providers (ISPs) and private networks.

Windows XP silently disconnects any active connections before entering standby. When the computer resumes from standby, you must manually reconnect any remote access connections before you can use them.

This behavior is by design.