There are three flavors of trunking protocol: Message, Transmission and PTT-ID.
In Message Trunking, the assigned voice channel remains up for the hang time once the initiator unkeys, and any unit sending valid freq and connect tone on the voice channel input can transmit over the voice channel, either during the initiator's (or anyone else's) transmission (a barge in) or during the hang time. Pressing the key on a subscriber that has made the initial switch to the voice channel will transmit on the voice channel input until expiration of the hang time.
In Transmission Trunking, the voice channel is disestablished as soon as the initiator's radio sends the disconnect signal. Any replies require a new channel grant. Pressing the key on a subscriber that has made the switch will not transmit on the voice channel input, but will send a fresh channel grant request ISW on the control channel input.
When these were the only two options, public safety almost universally opted for message trunking, since the capacity to barge in is considered potentially a life safety issue for police and fire use. However, message trunking does not allow the system controller to see the IDs of any units other than the initiating subscriber.
As a result, Motorola came up with a third flavor, PTT-ID trunking. This functions the same as Message Trunking, except that the non-initiating subscriber keyed after initial switching to the voice channel first sends its ID on the input side of the control channel, and then it switches back to the voice channel and accesses the voice channel via valid freq and connect tone. Barge in is permitted. Today, PTT-ID is universally employed in public safety applications.
For any given talkgroup, the conversation type set in subscribers' radios in the "personality" attached to that talkgroup must match the conversation type assigned to that talkgroup by the system controller.
The Motorola description of the foregoing is thus:
Message: The system assigns a traffic channel for the duration of a single transmission by one subscriber. When the subscriber dekeys, all subscribers remain on the traffic channel until the traffic channel hang time expires. Once the hang time expires the subscriber radios return to the control channel. Any radio involved in the call may PTT while it is on the traffic channel without returning to the control channel. Message trunking is available on 3600 baud control channel systems. It is not compatible with Motorola Astro 25 Systems
Transmission: The system assigns a traffic channel for the duration of a single transmission by one subscriber. When the subscriber dekeys, the traffic channel is deallocated and all subscribers involved in the call return to the control channel. Any subsequent traffic channel requests will be sent in on the control channel by the requesting radio.
PTT ID: Indicates the system assigns a traffic channel for the duration of the entire conversation as follows. After a subscriber dekeys, the voice channel stays active (hang time) during which time the subscribers stay idle on the voice channel. Any subscriber responding during this hang time will first key up on the control channel to send a message with its PTT-ID indicating it wants to send audio now for this call: the system then sends a grant message directing the subscriber back to the same voice channel acting as the new source.