I assume you are not talking VoIP - standard wired phone. Sadly, no, I have not seen anything that does exactly this. The closest thing that I know of (and use in the office) is a Plantronics Amplifier, model MX-10. It allows the shared audio, but has none of the cool automatic features and only supports mono at the headset (mixing the channels). You might want to check out the
GN Netcom 9120. It seems pretty close, but (again) only looks mono.
If you spot anything close, please post - I might be interested. Thanks!