Currently, when an iPhone user asks Siri to place a call or send a message, the assistant defaults to using Apple's Phone or iMessage apps. It is possible to place calls with other services such as Skype or WhatsApp, but the user has to specify which service they want to use in their voice command.
As reported by Bloomberg, once the new update for Siri arrives it will default to the user's most popular messaging app for that contact rather than to Apple's apps. For example, if a user frequently communicates with a friend via WhatsApp instead of iMessage, Siri will default to using WhatsApp when asked to message that person. Similar functionality for placing phone calls will come down the line.
Technology companies including Apple are facing increased scrutiny about whether their practices are anti-competitive, with the US House Judiciary Committee pursuing an antitrust investigation into whether the Big Four of Amazon, Apple, Facebook and Google manipulated online competition.