If you use Siri to add items on your reminders list, it helps to set a default reminder list so Siri will know which one to add items to when you need to create new reminders. You can do this by launching the Settings app from the Home screen of your iPhone, then tap on the Reminders section. From that point, you can tap on Default list and tap on the name of the list that you want Siri to default to when you create new reminders.