Assistant uses a feature called Conditional Call Forwarding to receive your rejected calls and connect you to our system.
You need to make sure your US Cellular plan includes the call forwarding feature.
I have Call Forwarding
1. Disable your phone's Wi-Fi connection.
2. Open your phone's dialer, dial *90*5595335694 and press the send button.
Note: Press the dismiss button when a notification appears on your phone. If the message states call forwarding activation was a success then you are done. If it failed redial the activation code.
3. Go back to the app and try to activate the service.