The WireShark packet sniffer normally doesn't support APCO P25. If you want to sniff P25 traffic using WireShark then you'll need to patch the sources and rebuild it. Tested and working on ubuntu 16.04.5
sudo wget -O - https://gist.githubusercontent.com/MaxwellDPS/bab40419e80bb1d2f940d7923d1e40a2/raw/38233899ec49e0416c4c63d6b7e2a2a59d7e85de/install.sh | bash