Apple said that later this year an iOS update will enable Siri to work better with third-party messaging apps. If you use the assistant to send messages, it will default to whatever message you use the most, instead of defaulting to iMessage or Phone. You can currently use third-party apps with Siri but you have to specify “Send this message via WhatsApp.”
For example, if an iPhone user always messages another person via WhatsApp, Siri will automatically launch WhatsApp, rather than iMessage. It will decide which service to use based on interactions with specific contacts. Developers will need to enable the new Siri functionality in their apps. This will be expanded later to phone apps for calls as well.