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

On CME the Push to Talk Service can be Configured in two places.

It can be configured under telephony-service to be global for all 7921 phones on a system.
or on an Ephone-template to be assigned for specific 7921s on a system.

The PTT service is activated when you depress the thumb button on a Cisco 7921 Phone.
What you do is add all of your 7921 phones to a paging group and then map the PTT Button to an intercom that triggers the paging group.

Under Telephony-service or the ephone template use the following command (case sensitive)
service phone thumbButton1 PTTH<1-6>

<1-6> is the button on the 7921 that the PTT service will use.

For a really good example of this configuration Tony Huynh from Cisco has created a video on demand. At the link below.
http://www.cisco.com/cgi-bin/front.x/wwtraining/PELC/main.cgi?URL=20141238466615%2E1693