Assign the line to the Auto Attendant through #206-7-10-1, if it's an ACS R3 or higher, set the number of rings before answering through #506-1-10-0, and set all phones to No Ring on line 10 through Centralized Telephone Programming (perhaps set it to delayed ring on your Receptionist's phone, in case the line rings and the Auto Attendant is not able to answer because of all ports busy)