On my desk, I have a three line set (Panasonic).
Line one is the normal PSTn dial tone from the phone company.
Line 2 is dial tone from a Aplio pro set. When someone calls from a aplio set overseas, rings on line 2
Line 3: Is connected to a Quintum A200 set. The phone number asociated with that port is the same than the PSTN, but on the VOIP world.
When someone is calling VOIP to my PSTN number, the Quintum routes it to my line three. When I see that line I know that is someone inside my Quintum network.
You could go as far as getting a samll office setup, with a controler box and several multiline sets.
Some, like the intertel GLX sells on ebay for peanuts.
I am talking 20-30 box for the controller box for 12 phones and 6 lines, that use to cost $500.00 dollars.
That way, when you are on the phone in line one, someone else on the house still able to use line two on another set, or viceversa.
You could do that with more than one two or three line sets.