Desktop Hardwired "Speakerphone" for iPhone

I am considering getting rid of my landline and using my iPhone for all my calls, business and personal. Is there a device out there -- either a dock or something I could connect to the iPhone through the headphone jack or USB -- that would allow me to simulate the standard landline speakerphone function? In other words, comprising a speaker (with volume control) through which I could hear the person on the other end of the call, plus a microphone through which I could talk to them?

I've done a 15-minute web search and have come up empty. As more people choose to move to having just a cell phone, you'd think there would be desktop devices that improve the sound quality of the built-in iPhone speaker and allow you to converse hands-free.

Any suggestions?

Thanks,
Giraut