windows 2000 services

sir!
i have windows 2000 server with active directory service pack 3 , ISA server 2000 and Mdeamon 6 insatalled.3com robotics is my external modem my problem is that after 4 or 5 hours message appear on my screen “cannot connect to phone entry error756:this connection is already being dialed” After this error RPC,SMTP and others services become stop.when i restart my computer then automatically services become start. sir please solve this problem
THANK YOU