Problem about configuring ports in RRAS

I am using windows 2000 Advanced Server and configure RRAS services for my remote users to dial in, but when I installed ISA Server on my windows 2000 server, no client can able to connect with my server machine when I saw ports in RRAS MMC there was no port for my modem, however modem is installed and working properly when I tried to configure it in RRAS ‘s Port section and error message appears that ports information can not be saved, please friends tell me that how can I solve this problem.