AASTRA DT4x2 for MX-ONE User Guide EN - Manual (Page 62)

During Calls
DT412/DT422/DT432 62
Push-to-talk button
There are two different types of push-to-talk functions that are set
by SIM card parameters in the phone. For both variants the Mute
key has to be pressed during conversation.
The first push-to-talk function can be described as a group call/
conference call. To enter the group call the members must press a
Soft/Hot key with a pre-programmed dial-code. If one of the group
members wants to get in contact with the others, he/she simply
presses and holds the Mute key to open the microphone. The
microphone is muted again when the Mute key is released. The
dial-code is set in the Soft/Hot key as a phone number, see section
“ Hot keys and Soft keys ” on page 100.
Note: When this function is enabled all calls started with a
Soft/Hot key will be using the push-to-talk function.
The second push-to-talk function is used in systems where only
one person is allowed to speak (not two persons at the same time).
The phone sends DTMF tones to the connected system when the
Mute key is pressed/released to open/close the speech channel.