There is a specification in the hardware that must support Wideband, HD, or TIA-920-B. All three terms mean the same thing which I believe means FM radio quality. Via software the phone must support the G.722 codec and so must your VoIP provider, but the call for the entire route must use G.722 or quality is compromised.
If your familiar with Snom, their older 3xx series can be upgraded very easily with this, the new phones are already enabled for Wideband:
http://www.snom.com/en/products/accessorie...rvoice-handset/
For Polycom look for "HD" support in the phones description.