Use the Application button
(on left side of phone) to start applications such as Push to Talk or other services on your phone.
Depending on how your system administrator sets up the Application button, you may be able to use it from the main screen only, or you may use it from any menu or service, or even when the phone is locked.
The Application button can also be configured to start an application immediately after you press it or only after you hold it for several seconds.
For more information, contact your system administrator.