Talk back is already on. Give your party leader role Priority Speaker, and have each leader set up push-to-talk with two keybinds: normal goes to their squad, priority goes to their squad and the caller. It needs the Discord desktop app.
Server setup#
There is nothing to set on the dashboard. In Server Settings → Roles, pick your party leader role and turn on Priority Speaker. To limit it to certain channels, leave the role alone and allow Priority Speaker for that role on those channels instead.
Each leader's setup#
Push to talk
In User Settings → Voice & Video, switch on push-to-talk and raise the release delay a little so their last word isn't cut off.
Normal key
In Keybinds, add Push to Talk (Normal) and record a key — for example mouse 4.
Priority key
Add Push to Talk (Priority) and record another key — for example mouse 5.
Now the normal key goes to their squad, and the priority key goes to their squad and the caller. Other squads don't hear it.
Several leaders at once#
If a few leaders press their priority key together, their voices are mixed so nobody gets cut off. If it is too loud for the caller, they can right-click the Callout bot and turn it down. With voice teams on, every caller in the shot caller channel hears talk back.