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Press and hold the Voice button on your steering wheel or the Home button on the CarPlay screen to launch Siri. You can also make a call by tapping the Phone button on the CarPlay display, but this is not recommended while driving. If there are multiple people with that name in your contacts, you'll be prompted to clarify who you want to call. The call will play through your car stereo speakers. This will hang up and return CarPlay to whatever you were doing before the call.
Launch Siri. Say "Call Name" or "Call Phone Number" and wait for Siri to dial. Complete your call using your car stereo. Press the Hang Up button on your steering wheel or CarPlay screen to end the call.