From the ToS:
(c) Siri. If your iOS Device supports Siri, which includes the dictation feature, these features allow you to make requests, give commands and dictate text to your device using your voice. When you use Siri, the things you say will be recorded and sent to Apple to process your requests. Your device will also send Apple other information, such as your first name and nickname; the names, nicknames, and relationship with you (e.g., my dad) of your address book contacts; and song names in your collection (collectively, your User Data). All of this data is used to help Siri understand you better and recognize what you say. It is not linked to other data that Apple may have from your use of other Apple services. By using Siri, you agree and consent to Apples and its subsidiaries and agents transmission, collection, maintenance, processing, and use of this information, including your voice input and User Data, to provide and improve Siri and other Apple products and services. If you have Location Services turned on, the location of your iOS Device at the time you make a request will also be sent to Apple to help Siri improve the accuracy of its response to your location-based requests. You may disable the location-based functionality of Siri by going to the Location Services setting on your iOS Device and turning off the individual location setting for Siri. You can also turn off Siri altogether at any time. To do so, open Settings, tap General, tap Siri, and slide the Siri switch to off. You may also restrict the ability to use Siri under the Restrictions Setting.