There are different ways to do that.
One, is to have the receptionist ephoen to have mutiple DN, one for the connection PLAR, on for internal calls. Or, a single DN with a secondary number.
This way he/she can set the desired ring via User Preferences with Menu button on the ephone, for each line you assigned. There are special separators to do that when you tie DN to the ephone with the button command.
Finally there is a ring command under ephone-dn that you can use to set rings for primary secondary number.