Windows 2003 Server - RAS - Modem not connecting

We are experiencing a problem, which occurs on several servers using
different modems.

We have a modem connected to the system, with Routing and Remote Access set
up as a remote access server.

The modem seems to answer, but doesn't connect to the system. Sometimes it
will connect, but then if you hang up and retry it then won't.

This is using standard Windows 2003 server on an HP ML330 fileserver. I
have tried several modems including Zoom (internal and USB versions), 3Com
US Robotics external and Multitech MT5634ZPX-PCI internal modem.

I'm pretty certain that it's not the modem thats causing the problem, but
something in RAS.

Has anybody else experienced this problem, or better still got a solution??

Regards
Simon