Below is a white paper on Cisco's site for configuring the 7921 with the integrated wi-fi in the UC520.
7921 Phones in SBCS Environment Configuration Guide:
http://www.cisco.com/en/US/products/ps7320/products_white_paper09186a0080973d69.shtml#config_uc520
To configure push to talk, see: a video on demand configuration from Tony Huynh (Cisco) at the link below.
http://www.cisco.com/cgi-bin/front.x/wwtraining/PELC/main.cgi?URL=20141238466615%2E1693
