So I've been really using my Cisco 7921 phones a bunch lately. In fact they are quickly becoming my favorite.

I thought I'd save anyone who has never ordered a CP-7921 phone a little time and frustration. When part # CP-7921G-A-K9= is ordered it does not include the battery or power charger.

Wow Push to talk works on the 7921 out of the box with CME!  All because (unlike Call Manager) Express has built in paging.

We are currently in the planning stages before purchasing a UC520.

I found a great white paper on Cisco's site for configuring the 7921 with the integrated wi-fi in the UC520.
It is Titled :
7921 Phones in SBCS Environment Configuration Guide