Dial In problem for windows server 2003 R2

I’m just installed a USB 56K modem on a server 2003 and this server is a child domain.

I have create accept incoming connection. Create a user account, assign dial in permission to allow dial in and start the Remote Access services.

When dial from outside, type in User ID and password. It still shows me authenticate fail
What else should I do in order to dial in to server 2003 from outside thru modem?

I?m do the same things on an xp machine and try to dial in from outside to xp machine, login successfully.

Any help will be appreciated
Thanks.