[USR-TC] Lost Carrier Terminations

So I'm looking at my RADIUS Accounting logs tonight. I see a lot of "Acct-Terminate-Cause = Lost Carrier", is this normal? Keep in mind, I have about 100 total users, max about 15 simultaneous, and I'm seeing about 250 (about 10%) occurances of this in 6 days. A LOT of the sessions are 11-30 seconds long. Is this normal? Do some modems just do this on disconnecting, or are there really that many people just losing their connections when they didn't want to??? Any ideas? Thanks, Joel