Verizon is now offering Push to Talk service to regular smart phones for an extra fee.  5 dollars per month. See the following features.

* Make quick exchanges.
Push one button to contact team members or clients.

* Place group calls.
Hold conversations with as many as 50 total participants.

* Check team member status with Presence.
See who is available for a Push to Talk call.

* Manage contacts for the entire company online.
Add, delete and assign contacts, create groups and more.

* Use one number, one device.
Make and receive voice calls without needing a second device or phone number.

* Enjoy coast-to-coast coverage.
Connect on our nationwide broadband network, and get expanded coverage across the United States within the National Enhanced Services Rate and Coverage Area.

More info can be found here