Don't know about any that will tie into the existing audio system but there are many good wired or wireless Bluetooth handsfree car kits out there. Their operation is a function of the phone and not of the carrier. In other words, it does not matter if you have verizon or cingular, but your phone must have bluetooth to work with a bluetooth set.
Earlier today I put together my plans for a Handsfree Speakerphone for my Murano. Here goes:
In my pre-Murano days, I had a 2003 Avalon. Had a great handsfree car kit speakerphone from Nokia designed to work with their phones.
For my Murano, I plan the following:
Nokia CK-7W Handsfree Car Kit:
Bluetooth or Wired. Also, has a (steering wheel, I think?) remote for most Nokia Bluetooth phones.
List price is $179.99. I just ordered this unit from Malcom Wireless. Price shown on Malcom web site is $139.99. But, if you search for CK-7W on www.pricegrabber.com, and select Malcom, the price is $119.99 + shipping.
I will probably pair this up with a Nokia 6682 Bluetooth Phone, which has had great reviews.
I've had very good luck with Nokia products. I'll keep you posted.