Radio convars are placed in the server.cfg file located in your main directory. With these in the server.cfg, you are able to change the animation (from the default) during PTT.
For more informaion on FiveM server convars, see this CFX document. To browse GTA5 default animations that can be utilized in the below convars, visit this page.
setr scnp25:anim:aiming:anim "mic_chatter"
setr scnp25:anim:aiming:dict "mp_facial"
setr scnp25:anim:radio:anim "generic_radio_enter"
setr scnp25:anim:radio:dict "random@arrests"
setr scnp25:anim:vehicle:anim "generic_radio_enter"
setr scnp25:anim:vehicle:dict "random@arrests"
Server version must be R06.15.0 or higher for this feature.