Yes to needing both cables. The two cables serve as translators to adapt their respective hardware devices to a common connection. The cables will each plug into a female-to-female RJ-45 butt connector.
I have a Jabra, not Plantronics headset. The sound quality on mine is superb, since my headset is engineered to support wideband audio, which the OBi IP phones also support. I don't know about your specific Plantronics, but it will be the best sound you've ever had with that headset and any other phone.
