Radius Log authencate but error 691

Dear All, I have try to implement freeradius in RHCL 4 with my sql. My data base connected to the radius server properly and my radius server authenticate the user properly inside the network and outside the network. My problem is that when I try to connect any user through dial up connection my NAS and radius response and password authenticate(basically I using CHAP password for authentication) but modem shows error 691 in Windows XP O.S. Please any one help me with providing a suitable solution. I sent all the configuration file and radiusd –X out put in zip format. Thanks & Regards Amritap Sinha