We believe we have fixed this issue. Try the following:
1) Highlight State of Decay 2 and press the Menu button;
2) Go to "Manage game & addons";
3) Ensure you download the Doomsday Pack from that menu, not from the Store.
4) Launch State of Decay 2: Juggernaut Edition;
5) Attempt to call in the Ultimate Edition radio calls.
Please let us know if this does not resolve the issue for you.