Well, it's a lot simpler than VoIP. It does not have all the session iniatiation protocols and functions defined in the SIP protocol, neither the complex h.323 session management and interface to the public network (POTS).

Some solutions are actually just a small program installed on a smartphone (Symbian or Windows Mobile based), that put togeter a presence management IM-Style (some even based on MSN, Yahoo!, ICQ or AOL AIM), and allow users to press a button and record a message - when the button is released the message is encoded and sent over GPRS or CDMA. This kind of solution is available for free (Agile Messenger) or for a fee (FastChat) and we don't need operators telling us what to do. This solution adds some latency to the whole conversation.

Other solutions (like the one deployed by Nextel) use the network capabilities, and it's more sophisticated, with no latency at all, or very little depending on network load.

Short: you can do PTT now regardless of operator if you have the right handset and software.