XP Modem Occasional No dial tone

Intermittently, after disconnecting dial up connection to the Internet
(successfully confirmed disconnect), the phone line will loose dial tone.
This may be discovered sometime later when attempting to make an outgoing
call. Shutting the PC down resolves the problem. Any ideas?

Outlook Express is configured to automatically check for new messages every
(X) number of minutes. (And it is also configured to establish a connection
as necessary.) So, when it tries to check for messages, it [therefore]
tries to "reach out" to your ISP.

That's MY suggestion, anyway.
Please post a follow-up.
Ron