This one is tricky, there is no real difference between voip headsets and regular headsets that i have seen. The plural would be much better for selling items.
It might be useful for SEO purposes, but like "dsl and "adsl" no one really cares what it is called - it is just "have you got the internet?" . . . "have you got a phone headset?"
Maybe worth a few hundred to someone, but there is virtuall no voip headset market as yet, and there hasnt ever been a very big phone headset market.