From Polycom documentation
Group Paging enables you to broadcast one-way audio announcements to users who are subscribed to a specific page group. Push-to-Talk (PTT) is a collaborative tool typically used with wireless handset phones, enabling you to exchange broadcasts with users subscribed to a PTT channel, much like a walkie-talkie. You can transmit pages and PTT broadcasts using your handset, headset, or speakerphone and you can reject them, place them on hold, and end them at any time. Both Group Paging and PTT are available on all phones that use UC Software 4.0.0 or later.
In sipXcom it is possible to configure PTT and group paging for a phone or group of phones using 4.0 firmware and later. For this, in the phone's page go to Push-To-Talk and Paging Groups. You will find the parameters to configure PTT and group paging.
A comprehensive description of the two features and their configuration and usage can be found here: http://support.polycom.com/global/documents/support/technical/products/voice/Audio_Packet_Format.pdf