As it stands, whenever you attempt to make a call or send a message with Siri, unless you specifically mention an app, by default Siri will choose the iOS Phone app or Messages app. This is fine if that was always your intention to use those apps, but in case you wanted to use a different app, you couldn’t, at least not by default.
However, according to Apple, they told the publication that in an upcoming update to iOS, Siri will soon default to your most used app. This means that if you use an app like WhatsApp to frequently message a particular contact, it will automatically default to WhatsApp if you want to message that person using Siri. The same will apply to phone calls.
This is actually a feature that has sort of been available on Android for the longest time ever, where users could always pick the default app that they want to use for messages, calls, browsing the internet, emails, and so on, so it’s nice to see it arrive in some form on iOS.