You have set Idle seconds before hanging up to zero in Dial-Up Networking properties for both User preferences and Logon preferences, but SBS is dropping an idle connection after x minutes.
The Phonebook entry that you use may have been configured with an IdleDisconnectSeconds entry of x minutes. If you used the Internet Connectivity Wizard (ICW) in SBS, it sets IdleDisconnectSeconds to 300, which is 5 minutes.
To resolve the problem:
1. Open Winnt.sbs\System32\Ras\Rasphone.pbk in Notepad.
2. Locate the entry (\[ISP Name\]) for your ISP.
3. In the \[ISP Name\] section, locate the IdleDisconnectSeconds entry and set it to the value you desire.
4. Save the file and exit Notepad.