dial-out problem (12207)

hi

I have an ISA server with secureNAT clients.
ISA connects to internet with dial up (primery route) when a user 
requests a web page . the dial up connection was established correctly 
but I recive this message in clients internet explorer:

500 Internal Server Error - ISA Server dial-out connection failed. The
administrator should manually dial the specified phonebook entry to
determine if the number can be reached. (12207)
Internet Security and Acceleration Server 

please help me
tanks.
mahdi

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