If your phone is able to respond to "Hey Siri" or "OK Google" without the car kit, then you can press and hold the call button for 2 seconds to activate Siri/Google Assistant.
If doing this doesn't work, you will need to set up Voice Assistance on your phone first:
Please make sure your smartphone is CONNECTED to WIFI or WLAN/Mobile Data and also paired and connected with the Roadtrip, then follow the steps below according to your phone(Android/iPhone) to enable Google Assistant/Siri on your phone first.
After completing the above, simply press and hold the dial like a button for 2 seconds and you will hear a prompt tone indicating that the voice assistant is activated. Now you will be able to dial a phone number with your voice through the Roadtrip's microphone.
Note: If your voice assistant is not activated on the Roadtrip, you'll need to use the phone's microphone to ask for a number to be called.