Yes, House Phone is what you are looking for, but make sure to let them know then they MUST use the Special (sideways 8 ) key to access any feature, or press the Intercom or Call key to dial an extension (you will have to add it to the keymap).
On each extension:
House Phone: Yes
House Phone Day: 8
House Phone Night: 8
In this case 8 is the typical outside access code, which actually access the Outgoing Extension defined in Associated Extensions of the device, but you could put in a specific trunk extension or ARS code if you choose.
House Phone was originally intended for like a lobby phone or ring-down line when you go off-hook it just rings immediately to a specific point, but it can be useful for other things as well.