If it is properly configured, and even then, it is always on, but there are a couple of provisos. It is worth making sure everything is set up properly if your calls are not working the way you expect. There are a few steps you can take if it is still not working right.
If you are wearing headphones or earbuds, take them off and try talking into your phone. If you have a problem with a virtual assistant, try out your earbuds without any intermediate audio devices.
If you were recently using headphones or earbuds, make sure they are not connected to your phone. Sometimes an audio device will try to listen for your voice and play audio from across the room, even though it won't lose its grip on your phone. If there is a symbol in the Volume Slider, you can use the Control Center to see it. If there is, that device will probably be used to route the voice recognition software. Put it back in the charging dock. Start the settings app, tap the i to the right of the headphones in the device list, and then turn off the audio.
It is possible that "Hey Siri" is disabled. You might have forgotten to turn it back on after turning it off. If needed, it is easy to check and re-enable. Go to the settings app and then tap on the search button. At the top of the screen, make sure to listen for "Hey" and allow "Siri when locked", with the buttons swiped to the right. Press Side Button can be turned on if you want.
If it does a poor job of responding to your requests, it might need a quick retraining session. When it knows what your voice sounds like in the environment you use it most often, it does its job. You can start the settings app. Turn off Listen and then turn it back on again. When it comes back on, you should complete the short setup process for your phone, which includes speaking a few commands into it.
If you have a problem with your phone's microphones, you might be able to hear and understand with the help of the computer. Make sure that your microphone is not blocked by a poorly fitting case, screen protectors, dust, dirt, or lint. You can test the microphone to make sure it works and that it isn't the cause of the problem.
You might not know that the only thing that is on is mostly. If the screen is covered, the phone won't respond to "Hey Siri", even if you set it to always respond. Go to the settings app, tap accessibility, and then tap Siri. In the spoken responses section, you can always listen for "Hey Siri" by touching the button to the right.
Are you still not working? It is possible to resolve a software glitch with your phone by turning it off and then back on again, which will reset the memory and clear out the software that might be problematic. Simply restart your phone and then check your voice.
If you have a few nearby devices and the wrong one responds to "Hey Siri", that can be annoying. You can find that more than one device responds at the same time. The device that hears you the most clearly is the one that should respond exclusively. It is almost certain that one or more of those devices will be turned off if that is not happening. Make sure that there is a fix for this problem.
If you want to make sure it is enabled on an iPad, you have to start the settings app, tap the button and then make sure it is enabled. Click the Apple icon at the top left and choose System Preferences to make sure the status reports are on. Click if it isn't.